It's your personal lifetime of experience saying "if it looks like a duck and quacks like a duck..."
Tumblr is a for profit company like many of their peers in the social media industry. They will latch onto anything of perceived value and suck it dry for their own gain then toss it aside when no longer useful to their profit margins
A non-zero number of these companies are opportunists looking to fediwash their profile (just like ecowashing, pridewashing and so forth. Hell even Blueksy, the media darling of "we're fedi we promise!" is already engaging in Blue Checks(tm) and ads in streams. If it looks like a duck and quacks like a duck...
Specifically, what is Tumblr really worth without all it's advertising and web controls to limit content viewing without forcing an account? (hello, Instagram) Adding bidirectional ActivityPub to Tumblr would basically render the site useless, who would subject themselves to that trash heap of a website when they could get the actual content from, say, a Lemmy instance? s/Tumblr/Reddit == samesame
For profit companies who have built walled gardens and wish to stay that way (reddit, tumblr, meta, linkedin, etc.) are at ethical odds with the ActivityPub world, insofar as people are trying vehemently to get away from these companies. But you'll notice for profit companies who are within the ideals of Fedipeople (for example Nitro, or Tuta, or Proton, or Mullvad) do not get griefed due to alignment of values and morals of the communities in question.