Every one is entitled to define terms like "stalemate line" as they choose. But Dip players almost from the beginning have used it in its strict sense to mean one specific thing (sorry, I've already quoted this once today somewhere else!)
In the Diplomatic Pouch's
The Gamer's Guide To Diplomacy: Stalemates Rod Walker defines it as-
A stalemate is a position on the board which prevents any further advance by the enemy. A stalemate line is a line of units, none of which can be dislodged by any combination of opposing attacks and supports.
That can be more than one power on either or each side of the line if they are in alliance. The point is that there is no set of moves which can force a way through the line if defended properly. There may be such lines in Ancient Med but I've never heard of one nor seen one. The one you discussed earlier in relation to one recent game Petunia wasn't a stalemate line - sorry, I don't have the link. But in that game as in many others claimed as "stalemates", the side which could break through chose not to.
I'd really like to see an example if there is one.