A former Lifehacker editor noticed a big problem after switching from iOS to Android:

I recently switched from an iPhone to Android, and discovered shortly thereafter that my phone number was still associated with iMessage, meaning that any time someone with an iPhone tried texting me, I’d receive nothing, and they’d get a “Delivered” receipt in their Messages app as though everything were working as expected.

iMessage purgatory | Adam Pash

Make sure to disable iMessage before switching from an iPhone to something else, although I’ve seen some reports indicating that even that isn’t guaranteed to avoid the problem.

Is this what they mean by saying you can get trapped in the Apple ecosystem?