This is why if you’re telling someone something they don’t want to hear it’s best to only give one reason.
Senior dev overleveled and performing at junior level is one thing. Personal issues impacting performance is another. They have totally different solutions and just mentioning them together sounds like the manager has an axe to grind.
it may be true that these have different causes in a particular situation, but given the variability and length of employment in most companies and projects for everyone involved (managers and programmers) I would think it more likely that a manager wouldn't actually have enough data to reliably separate the two conditions when dealing with it.
That's a pretty weak excuse that would raise serious concerns if one of my managers said it to me. The job of a manager is to debug these kinds of things, and frankly, distinguishing between technical gaps and personal issues is the easiest form of this. That's not to say you never get it wrong, but you listen, adjust and course correct.
Senior dev overleveled and performing at junior level is one thing. Personal issues impacting performance is another. They have totally different solutions and just mentioning them together sounds like the manager has an axe to grind.