I'm not sure where the phrase "arguably treasonous" came from, because the YouTube page has been removed due to a copyright complaint. But in context it's pretty clear, and given past discussions on the concept of treason applied to political opponents, I'll tackle it.

What Limbaugh is doing is probably not, technically, treason, since he's not giving aid and comfort to America's enemies. Unless you want to dig into the question of whether RightWing revenants and dead-enders are setting themselves up as America's enemies these days in their bitterness over losing the election so badly...

A better label for Limbaugh's behavior is probably "traitorous", which focuses on disloyalty and faithlessness, two qualities that Limbaugh and his ilk have in abundance. Some definitions of "traitorous" describe the traitor as committing treason, but I don't think that treason is an integral part of the definition of traitorous. It's sufficient to make Limbaugh a traitor that he wishes ill upon America.

The question of whether Limbaugh is commiting sedition is an interesting one. The hard definition of sedition seems to require action, insurrection and open rebellion; but "seditious" is a tendency in that direction, and the promotion thereof, and I'd say that Limbaugh is, therefore, guilty of seditious behavior, if not out-and-out sedition.

I think that a reasonable person would agree that Limbaugh's comments are both traitorous and seditious; he is clearly loyal only to his ideology and just as clearly disloyal to America. But what do you think, Tom?