I think GP means that the projection used appears to be an Azimuthal equidistant projection [0] - which doesn't really make it a 'flat Earth map' as such, but if the Earth were flat I suppose it would be the only logical (non) projection.
Well presumably there's no accepted concept of a 'pole'. (If there were, yes I'm sure the Antarctican Flat Earth community is adamant it is the inverse of the above link..!)
But, generalising, 'where is the centre', I don't know.
Back in the day they presumably didn't claim to know, because obviously nobody had reached the edge, so there was just known area, and unknown beyond it, in every direction.
The Charlie Stross story "Missile Gap" has a literal flat Earth and just that "and unknown beyond it, in every direction"... The 'Earth' there definitely doesn't have poles.
[0]: https://en.wikipedia.org/wiki/Azimuthal_equidistant_projecti...