I think this is supposed to establish that the Doctor and Ruby have a similar approach to adventuring to Clara and the Ponds (latterly) of coming on adventures but then returning only shortly after they left and having a "real life". Plus I think this is supposed to not be their second adventure, but just the second one shown.

I'm not sure how well this come across to the "new" viewers the new Disney+ version of the show is supposed to be introducing, though.