This marks the first time Peter Purves, Frazer Hines and Wendy Padbury return as Steven, Jamie and Zoe onscreen in years. Though they already returned to their roles in audio dramas for Big Finish Productions many years previously.
According to Russel T Davies, these events are actually happening in the Whoniverse. The memory TARDIS floated away into the time vortex after the Fifteenth Doctor got the last bit of use out of it. It is keeping itself alive by collecting past occupants of the original TARDIS in order to collect their stories.