That is only partially true.

For one, they could make a new sub, which people have been doing for like a decade, long before Lemmy was even thought of. Most of us know of at least one sub that got its start this way, before we had ever heard of it (or rather, we used to, before we collectively chose to forget all that!:-P).

For another, people can migrate off of Reddit entirely. There are multiple options to choose from, the top ones being Discord, Bluesky, even X, and as always Facebook, Insta, Tiktok, YouTube, etc.

All of these are exceedingly poor choices for various reasons or another... however, if history shows us anything, it is that people will do ANYTHING other than come to the Fediverse. Just read pretty much any Reddit post discussing Lemmy and you will see words like "leftist trolls", "extremists", "toxic AF", etc.

Also remember that even Reddit offers more freedoms in some ways than Lemmy. Examples include the ability to ask questions of the mod team via modmail without having to individually DM each (most often inactivated) account in turn, the ability to continue conversations begun on a post that is later removed by a mod from the sub's feed (Lemmy instead gaslights you, pretending that it never existed and you are crazy for attempting to return to it), and the ability to not continually receive notifications via DMs or comment replies from toxic trolls (ahem hexbear ahem lemmygrad ahem, cough), without having to individually user-block each account one by one by one by one by one by one by one by one by one by one, and truly almost infinitely since they keep spinning up alts all the time.

If we offered a VIABLE alternative, then perhaps they would come here. But when faced with man vs. bear, they chose The Man (we here are the bear, I am sadly coming to realize).