Film cans? I thought the whole reason the series was missing was because it was shot on video, and then the tapes were wiped after shooting?

You're correct but as Uvix has said, BBC Enterprises made film copies for overseas sales before the original tapes were erased.

The earliest episode to survive on its original videotape is Ambassadors of Death episode 1 from 1970. None of the original 60s tapes still survive, though I believe there is at least one tape that we know used to have Doctor Who on it but which now has another programme.

The earliest episode to survive in its original medium is possibly The Dalek Invasion of Earth episode 5 (The Waking Ally). That's because, while this was shot on electronic studio cameras as usual, there were no videotape machines available to record.

Instead the output of those cameras was telerecorded straight to 35mm film. AIUI the negative of that telerecording still exists.

The BBC used a kinescope much longer than the US. (A kinescope recorded TV to film.)

The US pushed a lot harder than Europe for videotape because kinescopes dropped frames off of American 60i frame rates, but worked really well for European 50i frame rates. Thus the BBC continued to use kinescopes for a long time.

Film prints were made for overseas sales.