Yes, to be a counter, a Pokemon does not have to be able to switch in multiple times. Hippowdon is almost universally agreed upon as a counter to SD Lucario, but he cannot switch in multiple times. Lucario uses Close Combat on the switch, and then Hippo is in range to where Lucario can SD and KO it next time. Now are you going to tell me that Hippowdon doesn't counter SD Luke? It can't switch in multiple times, but it is still a counter. If what I said "means nothing in real play", then the concept of a counter also "means nothing in real play".
Of course having Virizion on your team doesn't make you Landorus-proof. But having a Gastrodon on your team doesn't make you Starmie-proof either. Gastrodon can get Toxic'ed, or severely weakened by a mispredict, or whatever. Counter doesn't mean "Now I am safe from this Pokemon forever", it means you have a safe switch-in to that Pokemon who can do something back to it. Which Virizion does to Landorus.
My argument on this point was based entirely on principle, that multiple hazard switch-ins can basically eliminate the very idea of counters, and that's true. In the case of CB Scizor, you're right that 2 SR switch-ins isn't at all unreasonable, and is in fact a distinct possibility. However, our arguments are completely independent of one another (the proving or disproving of one does not affect the other) so I agree with you that there's no point in discussing it.
***Technically Scizor can switch in but only if Landorus has taken prior damage and is in BP range. It's not a counter because it can't just switch in whenever it wants, but it's not fair to say that Scizor can never switch in.