Ooh look, #OpenStreetMap. Sometimes. Not entirely sure the logic of what's used where.

https://youtu.be/CPCgXbuWiR4

99.82% of London isn't London

YouTube
@InsertUser
It is a slightly silly way of framing that the City of London is 0.18% of London.
@InsertUser Before even watching the video, I assume they refer to the administrative boundaries of the central municipality of London, in relation to the rest of the Greater London area.
Common issue with any large/metropolitan city when you want to strictly differentiate the administrative entity, which usually corresponds to the original place, from the urban agglomeration.
@jimkats I think the point, if there is one, is that nothing is officially called London except the City of London as the broader area is "Greater London" for some reason.