Yea, there are two countervailing clues in this, as far as I can see.
- They haven't done it yet and haven't talked about it since the promise. Additionally, a report linked here recently says they're losing money and people here commented that there isn't a clear way for them to monetise. So there just may not be any means or incentive to do so.
- As you say, their parent company is more active on activitypub integration. Additionally, Meta's Threads joining (maybe?) soon changes the game somewhat. Presumably playing directly in the same field as meta is enticing depending on their monetary incentives. Beyond that though, Meta have cited wordpress in their press releases on joining the fediverse and what that will mean for users, so you could speculate that Meta, who contacted and met with many admins on mastodon, also contacted and met with Auttomatic. If true, then Tumblr's federation plans, wherever they're up to, are no longer about the fediverse as it is now, but about integrating with Meta's Threads, which may explain the delay as the figure out how they want to go about that.