It doesn't matter that Swellow can OHKO Roserade if it can't switch into it ... thing about it this way:
Roserade switches in. Are you going to switch in Swellow? If you switch into Sludge Bomb / Leaf Storm, you are dead. If you switch into Sleep Powder and you haven't got Toxic Orb activated yet, you've lost a Pokemon. If you switch into Spikes, you've still taken 25% from SR on a Pokemon that cannot recover health even from Leftovers ... and that's the ideal case. You COULD say that based on what item Roserade is holding (if it has Leftovers or not) then you have a better chance of predicting what Roserade is going to do, but the fact is that even in the ideal case you are taking so much damage with only a prospective U-turn as compensation (or you could blindly Facade / Brave Bird, because Roserade is going to switch out).
We don't call Chandelure a good switch-in to Darmanitan because it can switch into Flare Blitz, we call Chandelure a good switch-in to Darmanitan because it's also immune to Superpower and 4x resists U-turn. Even then it's not too good a switch-in because it's vulnerable to EQ and Rock Slide + is weak to SR, and the two comparisons aren't completely equal because Roserade is considerably more versatile than Darmanitan (it also gets a second STAB). Swellow CAN switch into Roserade's Sleep Powder, but so much more of the time it will be doing something else. We say Primeape is a good switch-in to Darkrai not only because it can avoid sleep, but because it 1) resists Darkrai's Dark STAB, 2) has U-turn and 3) can threaten to outspeed and OHKO with Close Combat. Swellow achieves the latter two but (crucially) not the first.
We can say Registeel is a safe switch-in to Roserade because it's bulky enough to take sleep and has nothing to fear from any attack Roserade may use. Roserade can put up hazards, but it won't be immediately threatening to Registeel. So Registeel is safe. We CANNOT say the same for Swellow. You CAN say that Swellow is a good switch-in to Roserade once you ascertain it will not be attacking or using Sleep Powder (viz. it's low on health and you know it's going to Synthesis / Rest, or you know it's a defensive set and is laying down hazards), but you can't say Swellow is a good Roserade counter, or even check, because it cannot switch into Roserade much of the time.