Published a full game-ready asset that I made with a help of free hi-poly source from this stock. Am I infringing copyright?

Yes, you do.
You can only do that with permission from the original author (and obviously in this case you can't even be sure that the original author is the one that published it here).
https://www.cgtrader.com/forum/selling-buying-3d-models/why-your-account-has-been-suspended-here-is-the-possible-answer

Hi,
I don't item in question is copyrighted, so you can't infring the copyright. I guess you want to ask do you breach the license terms by posting your work. And the answer is NO. Your work is considered to be derivative of original and since all cgtrader's free models are shared under Creative Commons lincense (unless stated otherwise by author), you are free to use, remix and share derviative works, as long as you attribute the author. However there is small problem how you attributed the author - i think you did unsatisfactory job here and things certainly can be improved. Please take a look here and see how proper attribution should be made: https://creativecommons.org/use-remix/ If you'll fix this issue, then your work will be absolutely fine.

You are not allowed to modify, sell, or publish for free model that isn't yours.

@trimitek, @PhantomDesign guys, please do not spread false information - one is certainly allowed to modify and share the works based on CC BY licensed artwork. Resell is probably not allowed (i'm not entirely sure about that), but you are allowed to share such work for free under similar license.

I believe trimitek is right, you should listen to what he says.

I guess the conclusion the OP and everyone can make from this thread is to not rely on a information that somebody in a forum tells you is right (and I include myself in this).
Read all of the opinions, but do your own research, because in the end you upload the model and the responsibility is entirely yours.

Thank you guys for the informative replies!
@trimitek you said "The model in question is labeled with Royalty free license. And the only change I see is in a few colors/materials." - these are completely different products, coincidence is observed only in the shape of an object. Dmitri's model is detailed hi-poly mesh, mine is Unity's textured prefab asset with LODs.
I agree with @LemonadeCG that sharing, working upon, and attributing free assets should be encouraged by the community and not the other way round.
I'll contact tech support and ask about this dodgy situation.

@trimitek got it

@trimitek, to be honest, i find your attitude "everyone is a thief, unless one can proof otherwise" to be disturbing. SlashAssetStash did nothing wrong, nor illegal here. And even in unlikely event if original artwork would be found illegal, that would not be valid reason to terminate his account, or pursuit lawful actions against him. Most likely it would end up in removal of "original" and derivative assets without further consequences for SlashAssetStash. I can't say what it would cost to author of "original" work, as it depends on circumstances we can't know.

@LemonadeCG I think you are interpreting my words wrong.
I would say it's more like - 'prepare for the worst and wish for the best' kind of thing.
How likely or unlikely you think is something to happen doesn't change the fact that it can happen.
As you said, you don't know the circumstances.
You don't know what kind of person you may encounter and how good his lawyer is and you don't know what kind consequences it may have.

Why not produce something from scratch to a final product all by your own, maybe plan a hierarchy of it's parts and rig it, create own morphs and/or animate it and be darn proud you made it ?
No bother with confusing copyright laws then. ;) - Just my little input to the discussion.
- Over and out LOL...

So I read all of the comments on this post and decided to contact technical support and ask about this specific issue. But technical support guy didn't tell me any serious argument why I should remove it from my page.
Finally, I contacted the original author of hi-poly source and he told me that if I created low-poly model, textures and other stuff by me then he doesn't mind and it is OK to publish my game-ready asset version.

In conclusion, I want to say that as @trimitek told above there are different people and you never know what to expect from them. It is really 'dodgy' situation.
Otherwise, I fully agree with @LemonadeCG.

I just want to add something that is important and was left unclear above.
I had contacted support to clear it up and as I thought, all free models published on CGT are by default with Royalty Free License and not with Creative Commons license.
You should trust what is written on the product page.
That point 24. in CGT terms in condition is a bit misleading and should be considered more like a recommendation - if you want to set your free model to CC license, you should switch it's license type to Custom and write that down.