I’m going to use your post as a springboard, so please forgive my bloviating
This exact wording is what causes us issue.
There is muscle mass. There is lean mass. There is no lean muscle mass, because muscle is, by definition, lean. There’s no “fatty muscle mass” to contrast it.
But the fitness industry created this idea of “lean muscle mass” to sell stuff, and then it makes the dialog confusing.
Because, hell, you can put on 8lbs of lean mass in short order: drink a gallon of water. You step on the scale, you’re up 8lbs. None of that mass is fat, so it’s lean. Taa dah!
But putting on 2-3lbs of muscle? Yeah: that’s insane.
Going even further: are we talking muscle or “muscle”? Again: it gets confusing. Look at a steak

That’s a muscle, right? I even picked sirloin, so it’s “lean” muscle right?
Nope: I’m showing you a picture of WATER. Like, 90% water, and just a little bit of muscle.
You wanna see a muscle?
THERE is PURE muscle (ish). It’s almost entirely dehydrated down to nothing but muscle.
So if I say I put on 2-3lbs of MUSCLE, that’s 2-3lbs of beef jerky, and THEN, because I’m still alive, my body will fully hydrated that muscle, and I’ll put on something like 6-8lbs of scaleweight.
That’s nuts. That kind of growth is bonkers.
But 2-3lbs of scaleweight, correlating to maybe half a pound of muscle alongside a bunch of water? I can believe that.