I honestly don't care whether or not Swellow gets ranked however you seem to have just missed out on a lot of main points with this post and the main issue is the absurd amount of comparisons with Exploud which ill touch into in a moment. Something else that is shaky is that the venusaur you were facing was most likely defensive or carrying an Assault Vest im still shaky on the tier and i asked what spread were mostly used on venusaur and i was told that this was the main one.You know, I'm against Swellow being ranked at all. I played against it with a Venusaur at 51% health, and it did all of 45% with boomburst and then ate a sludge bomb. Hurray! The thing is, while fast Boomburst seems cool, reliably 2HKOing doesn't matter when you don't have the bulk to stick around to snag the 2HKO in the first place. 60/60/50 bulk is TERRIBLE across all costs, without even factoring the SR weakness in. Boomburst, while a great STAB move, only 2HKO's most threats at best and can't get the type advantage. Swellow also needs Specs to barely come close to Exploud's power [Base 50 Special Attack is terrible], but that removes the option for Coverage Exploud has to hit the likes of Mega Steelix without Hidden Power or locking yourself into a move. Well sure, being a faster Exploud with less power is cute, there's almost no reason to use Swellow whatsoever when Exploud is in the tier, due to Exploud eclipsing Swellow in every area but speed. Though you're trading Speed and a Rocks Weakness for power and versatility, so Swellow is never worth it.
252 SpA Choice Specs Swellow Boomburst vs. 0 HP / 0 SpD Venusaur: 190-225 (63.1 - 74.7%) -- guaranteed 2HKO
Something you also seemed to undersell is that speed. Swellow is only out-sped by three other pokemon, bar scarfers, in RU and that is Jolteon,Aerodactyl and Accelgor which is very impressive from a revenge killing standpoint combine that with the ability to 2HKO the majority of the tier with just slight chip is very nice especially considering you will be using U-Turn until comfortable. Yes Exploud has much better coverage with higher base power moves in Fire Blast,Surf,Focus blast however the moves Swellow has in its arsenal are well enough in Heat Wave and Hidden Power Water (Presumably) as far as roles are concerned i would only compare these two with that access to boomburst however that is all. Exploud is wallbreaker it is supposed to break common cores down with those powerful moves. Swellow is a revenge killer you are not going to come in after say Durant has killed something with Exploud its too slow and you would want to preserve its HP for when it is able to come in with little chance of it being worn down or straight up killed however Swellow is faster and has the ability to OHKO it with ease.
if it were to be ranked i would agree C or maybe C- is more fitting as the points to have this thing ranked at all are reasonable enough in my opinion.