Mozilla is unhappy because the use of browser engines other than WebKit will be restricted to the EU, forcing them to develop two different apps.
For an independent browser like Firefox, managing two browsers is not easy, so it can be forgiven that this could be seen as almost harassment.
Also, the fact that the use of browser engines other than WebKit is limited to iOS means that the use of WebKit is still forced on iPadOS, which also increases the effort for Mozilla.
No, it’s not a sunk-cost fallacy.
If you already have a bunch of Apple stuff, it makes more sense to continue using Apple stuff, because switching would cost money and effort. You’d also lose access to the software library that you paid for.
Having a bunch of Apple stuff also makes buying more Apple stuff in the future a better value proposition because you gain access to features that you wouldn’t otherwise have. Platform lock-in is not a sunk-cost fallacy. You’re just uninformed and being smug about it.
The sunk cost fallacy only applies when stopping is free or the cost is low enough (in money or effort) that it makes more sense to quit than continue.