Cohen initially testified he alone made the payments and trump never got involved at all - and Stormy wrote a letter saying none of it ever happened and Trump was nothing but a gentleman when they met. These are supposedly the “star witnesses” - an now it looks like they’re going after business interests which would be IRS-realm items - and the IRS has nothing they feel worth pursuing...
That’s what it looks like to me. Seems like it was a find anything we might be able to get him on a fishing expedition. As you said, if he did these things, the IRS would’ve already figured it out by now.
If they testified under oath that Trump paid them to commit perjury - that is a triable fact.