So I was able to check and that tube is effectively the vent. However, I'm unable to unscrew it because the seat's frame is in the way, so I had no choice but to use a stainless steel wire and ram it in to unobstruct it :-( The end of the vent was fine though, only when halfway did I hear a 'hiss' with the tank pressure being equalized.
I used then my shopvac in blower mode through the fill port to see if it was venting fine, and it is now. Only then did it occur to me that I should have put the shopvac BEFORE raming the wire in to prevent what was blocking from may be falling in. The filter should catch what fell in, if something fell in though.