Definitely. Meta is studiously only sharing the number of accounts registered. We have no idea how many of those are active. If we go with the old 1-9-90 rule, only about 10 million of those 100 million will become active users. Although, the rule obviously isn’t a universal constant. On the fediverse, for example, it’s closer to ⅓ of registered users that are active.
Meta is categorically evil, but the pretty obvious gain from federation is the same as it is federating with anything else: content. Threads has people posting on it, some of those people say interesting things … the end.
That’s not to say that outweighs the downsides, like some of that content will also undoubtedly be hateful bigoted trash, and the moderation load of suddenly dealing with an order of magnitude more posts will be a huge strain on fediverse admins who choose to federate, but there’s undeniably something to gain.