I like your explaination, Indeed the forbidden scroll which might of had the FTG inside being a S rank jutsu but Minato was using FTG before he was a Hokage so couldn't see the scroll. I remember hearing Minato made his FTG using what he learned about seals from Kushina. Tobirama is a Senjutsu, not all sealing jutsus came from the Uzumaki clan even if they are distant relatives.
Minato can also sense things using his finger on the floor like Tobirama which is not shown with anyone else. It's more likely that Minato was told stories about Hashirama, Madara and Tobirama and self taught himself the jutsus he liked. Minato thought Obito was Madara just from the feats Obito did but then said it doesn't matter who he is.