They can, if they exercise their right of ownership to their trademarks. After all, by accepting the d20STL, you acknowledge WotC's ownership of the listed trademarks and not contest nor challenge them.
You could always bring the argument to court.
If company A continues to sell products with the d20 logo on them isn't a challenge to WotC's trademark. Company A is operating under a license to use that trademark, and the terms of the license do not address revocation without a breach. So there is no challenge to the ownership since Company A is operating under a legitimate license.
But I think it'd be dumb to force the issue, especially considering how little that logo means now. I'm just saying that a forum post by a WotC employee isn't technically enough to end the license. On its face, in fact, it appears that WotC needs to update the license rather than revoke it.
But of course I'm not a lawyer, so there may be something
in trademark law in general that allows WotC to unilaterally cancel a trademark license. But, it is not a challenge to the ownership of the trademark. It would be a challenge to whether or not WotC can revoke the license rather than update it. Within the STL, there is nothing addressing that. In U.S. trademark law, I have no clue and don't claim to have one.
All I'm saying is that within the d20 STL itself, anything short of a d20 STL version 7 doesn't prevent currently licensed publishers from continuing to use the d20 logo. Plus, I'm mostly pointing it out for the sake of theoretical argument. I don't plan to publish any d20 logo material (although I did "download the enclosed graphics" back when they had the graphics on the site, so technically, I'm still licensed to distribute d20 material). It's just that if you read the fine print of the d20 STL, there isn't anything
in the license that allows WotC to unilaterally revoke it and therefore prevents publishers from continuing to sell d20 material.
Plus if I were a publisher that doesn't visit EN World, I would have no notice at all yet (I would be bad at my business if I didn't visit the top fan site, but that's a different matter.)
If nothing else, if I were a publisher I wouldn't pulp any books without consulting a lawyer first.