If we assume that the same opponent is in all the finals of the majors, that'll yield 4800 points. That means that same opponent will need to win four Masters 1000 a year to overtake the hypothetical CYGS winner. Winning those Masters shields will be a tough ask if the CYGS winner is in the draw plus all the other players who will be splitting the finite number of points available sans the majors. Just look at history and see how many players got to #1 just by winning one or two majors a season; winning three majors virtually guarantees the #1 ranking. The only "realistic" way that the CYGS player doesn't obtain the #1 ranking is if everyone else conspired to have the hypothetical runner up vulture all the points left and the CYGS winner sits out everything else.
I didn't say it was likely
. But the premise of this discussion is one record only, so nothing else should be taken for granted. 12 majors, certainly, that is implied. Weeks as #1 are not. Though anyone winning all the slams would certainly be regarded
as the best player that year even if someone else has more points.