[Retros] Is it always right to loose because time?

[Franco]

Hi friends,
let us suppose that after his flag's fall the player
shows to the arbiter that the only legal move
he had just was checkmate.
As example:
1k6-ppp5-8-KP6-P7-8-Q2r4-4b3

W plays Qg8 and then B flag just fall before he can move.
The only move he could do is just:  Rd8 mate!
The Question is: is it right that Black looses that game
because time?
I think no. I think Black is the winner.
bye
Franco