Hi! I'd like to ask about some little detail that has recently happened for the first time in my years playing MTG.

So, my opponent controls an [[Intruder Alarm]] and I'm hoping to tap their creatures before combat with [[Cryptic Command]]. They announce they are moving to combat so I respond "before moving to combat, cast Cryptic Command tapping your creatures and drawing a card" and the spell resolves.

At this point, I expect the game to move to combat, they can't declare attackers so they move to second main. However, they say "so, as I'm still in my main phase, cast this creature, trigger Intruder Alarm, untap my creatures, move to combat again".

Now, I know the most 'correct' way of doing it would have been to tap them at the Beggining of Combat step, before moving to declare attackers, but I'd like to take this as a learning opportunity. As I've always understood it, when you announce you're moving to a certain point of the turn and someone responds, the game still moves to that point automatically (checking priority again) after the fact. So, my opponent would still be moving to the Combat Phase without a chance to cast anything at sorcery speed.

I'd like to know whether I've been wrong all this time or not. Also, what's the ruling that specifically talks about this? I can't seem to find it in the Comprehensive Rules. Thank you in advance!