Can't quite agree with this.
Here are phrases about the target from Dread Counterspell and Counterspell:
DCs: "a creature he can see that is casting a spell"
Cs: "you see a creature within 60 feet of you casting a spell"
Cs: "a creature in the process of casting a spell"
Sorry, perhaps I'm no grammar wizard (College of Scribes?) but, to me, this difference does seem either small or sloppy or both.... or, another take is: there is no difference. If it were super clear, this discussion wouldn't be happening.
That said, please explain the difference, if there is one, between:
"a creature... that is casting a spell"
and either:
"a creature in the process of casting a spell"
or
"a creature... casting a spell"
(and, FWIW, I honestly still don't know 100% how I'd rule a Subtle Spell vs Dread Counterspell)