You know, if you move around the 'just' in this title, it changes the meaning of the title to almost every opinion I've seen about Mastodon and Twitter
Just Mastodon Isn’t a Replacement for Twitter
Mastodon Just Isn’t a Replacement for Twitter
Mastodon Isn’t Just a Replacement for Twitter
Mastodon Isn’t a Just Replacement for Twitter
Mastodon Isn’t a Replacement Just for Twitter
Mastodon Isn’t a Replacement for Just Twitter
Mastodon Isn’t a Replacement for Twitter Just
"how's the article coming along?"
"hold on, I'm twitterjusting it"
It's meant to convey a sense of defeat. So in this case the sentence would read as implying that the author wanted Mastodon to replace Twitter but that it's not able and that fact is causing her emotional pain.