That merely says that there is no plan that the writer knows of to pull the plug on you. And indefinitely is a duration of currently undefined length, not infinite length.
which isn't a promise that it will be "free forever". in the context of the rest of the article, it's not even an attempt to mislead you into thinking it will be free forever. though it is apparently enough to base willful misunderstanding on.
And their broader thrust in the post is less the forever and more of the "own your mistakes". Don't pretend like you never had a free forever plan.
Their promise seems to be less "we'll ACTUALLY be free forever" and more "we're free right now, but if that's going to change, we'll communicate with you".
> We fully expect to continue offering the Zulip Cloud Free plan indefinitely,