And yes, it's true that the governments could always mandate such scanning before. The difference is that it'll be much harder politically for Apple to push back against tweaks to the scheme (such as lowering the bar for manual review / notification of authorities) if they already have it rolled out successfully and publicly argued that it's acceptable in principle, as opposed to pushing back against any kind of scanning at all.
Once you establish that something is okay in principle, the specifics can be haggled over. I mean, just imagine this conversation in a Congressional hearing:
"So, you only report if there are 30+ CSAM images found by the scan. Does this mean that pedophiles with 20 CSAM images on their phones are not reported?"
"Well... yes."
"And how did you decide that 30 is the appropriate number? Why not 20, or 10? Do you maybe think that going after CSAM is not that important, after all?"
There's a very old joke along these lines that seems particularly appropriate here:
"Churchill: Madam, would you sleep with me for five million pounds?
Socialite: My goodness, Mr. Churchill… Well, I suppose… we would have to discuss terms, of course…
Churchill: Would you sleep with me for five pounds?
Socialite: Mr. Churchill, what kind of woman do you think I am?!
Churchill: Madam, we’ve already established that. Now we are haggling about the price."
Apple has put itself in the position where, from now on, they'll be haggling about the price - and they don't really have much leverage there.