It's an echo of the fondness with which she refers to them in PoA.

That really stood out to me in PoA, too. I happily subscribe to the fandom approach that has McGonagall as a former prankster in her own right who secretly applauds the Marauders' goings-on, even though as a teacher now she can't (much) let on. And I assume she has to try not to let them get the better of her, hence the frayed nerves in this scene as they kept her waiting, hee.

Glad for the smile. And no worries about communication, as always. ;)