Nope. If Facebook owns any patents for mechanisms that are used in Preact, they can sue you.
I'll say more: if you use Preact and there are such patents (I have no idea if there are any), they could sue you even if you don't sue them first, because Preact isn't covered by any patent grant.
If I were Facebook and a patent troll, I would closely inspect what Preact does and what patents I have that cover it and then go out and sue every company openly using Preact preemptively because switching from React to Preact indicates they might be planning to sue me in the future (and if I bankrupt them, I might acquire their patents this way).
That Facebook isn't doing this should tell you something.
I have no idea where this comes from but this doesn't make any sense whatsoever.
React comes with two things:
* a copyright license for the source code, which is a BSD-style open source license
* a patents grant license for "the React software distributed by Facebook" (i.e. official React releases but not third-party forks)
The former is unconditional except as defined in the terms of the BSD-style license itself. It does not magically terminate even if you walk up to Mark Zuckerberg and personally punch him in the face.
The latter is conditional and terminates if you sue Facebook, or if you get sued by Facebook and countersue over patents covering React. If Facebook sues you and you countersue them over a patent covering e.g. their social graph algorithms, you are still protected by the React patent grant. Only if you also happen to have patents covering React and decide to use them to countersue when Facebook sues you over other patents they hold, only then the patent grant is revoked.
Patents don't give a fuck about source code. Software patents are about abstract ideas. Source code is an expression of that idea and thus only protected by copyright. Software patents apply whether the patent holder expressed that idea or anybody else did.
If Preact does something the same way React does something and Facebook has a patent for that, you have no protection whatsoever. Even worse: Facebook likely already knows which of their patents apply to Preact or not, so if they wanted they could easily sue you for using Preact if they had such patents and felt particularly patent trollish that day.
You can't escape this. You don't know whether Facebook has patents for React. But you also don't know whether Facebook has patents for Angular, or Vue or Ember or even jQuery.
Angular, Vue, Ember and Preact are using the MIT license with no patent grant. Sure, if you're lucky, a court might say that the license implies a patent grant but there is no language in that license to hint that this is in any way intended (unlike, say, the Apache license).
Oh, and even if the MIT license would imply an unconditional patent grant, what makes you think the authors hold all the relevant patents? What if Facebook owns a patent for Vue? What if Google holds a patent for Ember? What if Apple holds a patent for React?
The reason you're upset isn't that React has an explicit patent grant with explicit limits. The reason you're upset is that you don't like software patents.
That's fair and I personally think software patents are bullshit, but raging against Facebook or worse the React maintainers over this and spreading FUD is not how you deal with it.
