Dan said that Disney offered to continue the show in several ways; with more seasons, as a series of specials, and as a mixture of specials and regular episodes like the fourth season. It's entirely possible that they hadn't actually made a decision on what to do when that report came out.
The show being outright cancelled doesn't really make a whole lot of sense considering the way Disney has handled the property since.
It kept a much higher profile on DXD than even many of their ongoing shows with it still getting yearly marathons and a considerable chunk of airtime every day, the main channel pretty much ignored MML until the crossover and spoiled Heinz's appearance in the promos, merchandise is still out there, and Disney+ marketing put a heavy focus on Phineas and Ferb from the start. Not a single one of their other former shows has stuck around in that way Phineas and Ferb has; even Gravity Falls has pretty much been relegated to late night/early morning timeslots. Usually when a show is done, it's done.
Either Disney had some big time buyer's remorse for letting the show go or Dan and Swampy's version of it being their choice is pretty much the correct one.