I posit the that Doctor invented Gallifreyan time capsules based on his comments "I was a pioneer in my day" (The Daleks, 1964) and, of the TARDIS console, "ever since I constructed it" (The Chase, 1964) (previous Time Lord time travel devices being restricted to stationary devices, timescoops and extraction chambers); and that this TARDIS is the first one ("one careless owner", Rosa, 2018).

The Doctor being on his second regenerative cycle is an allusion to The Brain of Morbius (1976) and is expanded on in The Hero of Three Faces dialog and annotation here.

The reason this cartoon - also, the one linked in the above paragraph - was removed from The Hero of Three Faces continuity is that the Doctor Who episode The Timeless Children (2020) created in screen continuity a different scenario for the Doctor having previous lives before the first onscreen Doctor, one that the Doctor does not remember and had not been aware of at the time these cartoons are set (though that doesn't preclude the possibility that the Doctor invented time capsules).