That's not the definition the others are using, but this seems to be a game of whack-a-mole with everyone's different ideas about what it means.
That said, I think your definition highlights the problem: By telling someone it's okay to feel the emotion, you've implicitly endorsed the response.
The situations I'm speaking about involve people developing inappropriate emotional reactions that lead to self-harm. When they surround themselves with people who do this "validate emotions" game, they're implicitly gathering consensus that it's okay to react that way. The cycle continues.
It's clear that a lot of people have picked up this idea of "validating emotions" being virtuous and good, but some times what people need is for people around them to explain that their reaction is not actually appropriate or okay.