It’s probably just a definition thing.
To me, constructive criticism means that the criticism doesn’t just point out failure, but that it then also shows how to correct that failure.
By itself, “you’re doing it wrong” is just destructive: it takes something apart, it destroys it. Without a subsequent “and here’s how you would do it right,” it doesn’t become constructive, it doesn’t help in putting things back together in the correct way.
Sure, as a first step, “you’re doing it wrong” is completely justified when something is actually wrong.
But without the second step - the constructive part - it just doesn’t constitute constructive criticism. By itself, it’s just criticism.
The reaction from Apple users is to blame Android users - which is entirely unjustified.
But of course, post purchase rationalization and brand loyalty play a big part in why people want to externalize blame rather than questioning their own decision or blaming their favorite company for providing a shitty cross-platform messaging experience.