I think the use of the word "sequel" was the direct follow up to the original film in a franchise. Civil War and IM3 are the third films in those respective franchises, unlike Age of Ultron and Guardians Vol 2. In that sense, GotG2 is indeed the second best sequel for the MCU at the box office.
Also, I think people who are claiming that GotG2 might be a "disappointment" are basically using a straw man. I don't really know what Disney "expected" or not in terms of box office results, but I think there is little chance that they are actually upset with the performance of the film at all. It's making at least $850M worldwide, probably will get to over $900M and that's a huge success for a film with a ~$200M budget and for this franchise in general. They've successfully monetized a group of virtual unknowns now for 2 films despite only have a mild connection to the rest of the MCU. They've already greenlit a third movie and plan to include the Guardian characters in Infinity War; the actions of the folks at Disney certainly don't reflect them being "disappointed" in the film.