It would have to be, CS2 is the fourth major installment in the Counter-Strike franchise.
Then again, all kinds of companies take liberties with naming including numbers. Look at Windows 7 (12th major release), Windows 10 (successor to Windows 8), the game Battlefield 2 (third in the series), Battlefield 3 (three games after BF2), Battlefield 1 (after the release of BF4), etc.
Half life 1, 2… hm.
Ok we’ll make three HL2 episodes to follow that up.
Ep I. Ep II. Uhhhh… and let’s stop there, just like forever I guess.
Portal 1. Portal 2.
Left4dead. L4D2.
Team Fortress. TF2.
Counterstrike. CS2. Oh shit we’re releasing a third one, we might have to use the number three finally, oh no… I’ve got it: CS:Go!