They still want to keep their reputation good (in the eyes of potential advertisers) if/when they want to add it in the future.

They show (or showed) continuous ads for Krisp.

Showing the trademark of a technology they are using in the product, for the purposes of identification of said technology does not count as advertising. According to your reasoning, they would have to say "we use some other company's noise reduction". And no, the chat view (or the voice chat view) does not show the Krisp logo continuously.

They showed the trademark very prominently and that must be because they had a deal. We give you noise reduction, you give us visibility. That is an advertising contract.

This doesn't follow, Mumble (open source voip) do exactly the same thing with the Speax codec in their settings