This is also not compatible with the ActivityPub spec but even if it were you’d win nothing because as soon as you fetch the post it is still on the server.
Mastodon actually just stores all this data on the server containing the post itself. Instance admins get as much information about the post as the client does. Both Lemmy and Mastodon use the same protocol, but Mastodon chooses to only to trust the server the user is using, and not the third-party servers.
I’d first have to create 2000 users, then I’d have to send 2000 upvotes. And then I’d get blocked by all instances.
Creating that many users wouldn’t be hard to do(you don’t need to use the GUI, just a little SQL is all that’s needed). And you don’t need to “send” the upvotes; you can sidestep the protocol entirely and just update the database. That’s the problem.
And while yeah, the instances would block me, they probably wouldn’t notice if I did it at a much smaller scale. In fact, there’s no real easy way to check whether these upvotes from an instance are actually real.