Hall: IBM, Red Hat and Free Software: An old maddog’s view

That's not how any of this works. Estoppel is an equitable right that attaches under a wide variety of circumstances. I am skeptical that it attaches under *these* circumstances in particular (at a minimum, you would need to produce a written record of the FSF and/or GNU explicitly and unambiguously endorsing a particular interpretation of the GPL, and even then it would likely only bind those parties and not e.g. Linus), but to assert that it never applies to copyright at all goes way too far. For example, a promissory estoppel might be styled as an "implied verbal license" or something like that, but the effect is the same: The plaintiff loses their case.

Tortious interference, on the other hand, is a matter of contract law. Since most copyright licenses are contracts, including the GPL in most jurisdictions,[1] it follows that tortious interference with a copyright license is possible, but again, I'm not seeing how it applies here. Vaguetweeting about how you don't like Red Hat's interpretation of the GPL can hardly rise to the level of tortious interference. At a minimum, you'd have to explicitly solicit RH's customers to breach their support agreements, and I am not aware of the FSF (or anyone else) actually doing that.

[1]: https://lwn.net/Articles/747563/