Doesn't mean you won't still have a challenge building up a presence again on a new platform, but getting to the point of equal engagement in a new platform where users are motivated and active only takes a tiny proportion of your followers to make the move.
Are we talking about mastodon or threads? Do you think Threads won't have an algorithmic feed?
It obviously will, so this point is only relevant to mastodon, which is still 100x smaller than twitter.
I think it is relevant to any new social media, because even with an algorithmic feed, unless it's really badly done, you'll first start to swamp out an increasing number of those you follow once you follow a large number and/or have given a strong signal about which of them you actually engage with, so you're likely to get a "honeymoon period" even with one where the need for a huge following seems less important because your followers are in that situation too.
You cannot move a 100k followers from twitter to thread. All the influential people HAVE to keep using twitter to serve their followers. Even if they build a following on thread, it will take years to build what they have on twitter.