Can figures with Power Cosmic TA have their abilities outwitted, ie Silver Surfers Hypersonic?

Nope. It's in the description of the TA.
They also don't take pushing damage.

Just checking as there was another thread with everyone talking about outwitting Surfers hypersonic!

They are probably discussing it in conjunction with Disbanded! (the BFC which ignores all TAs, even the Power Cosmic/Quintessence TAs) or just maybe another version of Surfer with HSS (the E from Super Nova, for example).

Or maybe they weren't realizing that there was the PC TA

In the Marvel Errata & Clarifications document, they specifically added that Power Cosmic could not be countered.

It's also possible that they were talking about using the Dissent feat and not knowing that about Power Cosmic?

Yes, they were.

There was debate about if PC could be "OW'd" with dissent... it was said that it couldn't, so they added that to specify.