This is not quite true.
To give a more detailed version of what Mayo was saying:
Rosewall: 4 Amateur Majors, 15 Pro Majors, 4 Open Majors + 2 WCT finals
Gonzales: 2 Amateur Majors, 15 Pro Majors
Laver: 6 Amateur Majors, 8 Pro Majors, 5 Open Majors
Federer: 17 Open majors + 6 WTF
Given reasonable weightings for different types of major etc., I think it's clear that Rosewall/Gonzales/Laver are not well ahead of, for example, Federer. Perhaps they are slightly ahead, or perhaps slightly behind, but it's certainly close.
Laver has a lot of titles which were even bigger than the AO/FO were in the 68-71 range and especially 68 where there was only 3 open era majors.
Examples include PSW in 68 and 70, Sydney Dunlop in 70, and Philadelphia in 70 all drawing better fields than the AO/FO. Additionally, Laver won the MSG Champions Classic in 70 and 71 which was far bigger than the FO/AO were those years as well as no one showed up for anything besides USO/Wimb in terms of traditional slams.
If we are going to weight dismiss Amateur Slams (as Mayo is doing, after telling me I shouldn't dismiss 500/250 titles and saying he ascribes value to them), we need to weight some of these as the equivalent of a Major, since they were top 4 titles in a given year.
1968 PSW was the 4th major replacing the amateur AO
1969 everyone showed up for all slams and Laver beat them all
1970 the Champions Classic in place of the AO
and either PSW, Philadelphia, or Sydney Dunlop in place of FO (take your pick Laver won all 3)
1971 the Champions Classic again over the FO.
So those 4 substitutions leave you with:
17 Majors each for Fed and Laver (8 Pro, 5 Open, 4 Open Equivalents)
You also have to value the 4th biggest event from 63-67 each as a major too since Laver only had 3 pro slams. Then in addition you need to value a 5th biggest event for each of Laver's years to be equivalent to the WTF and you also need to assign some weight to the Amateur Slams. Even, without doing any of that what you have is:
Fed - 17 Majors, 6 WTF, 21 Masters, 5 YE #1
Laver - 17 Majors, 0 WTF, 40 Masters, 7 YE #1
Mayo has said himself he weighs 2 WTF = 1 slam and 5 Masters = 1 slam
so by this you have the equivalent of 7 more slams for Fed (3 from WTF and 4 from Masters) and 8 for Laver (from 40 Masters).
That puts you at 25 Majors for Laver to 24 for Fed plus 2 more years as YE #1.
Additionally Laver has a CYGS at the pro level, amateur level, and open era level and this the bare minimum edge for Laver as you have not even done the adjustments for converting some of Laver's Masters level tournaments into Majors or WTF, as some of them would be the 4th biggest event in 63-67, or the 5th biggest every year of his career, or given any value at all to the amateur slams.
So taking a bare minimum level for Laver and using Mayo's own valuations of
Masters to Slams and WTF to slams (which are higher than I use), Laver still comes out with a lead, hence why I said he's well ahead because his lead could only grow from here.
Also I don't use 500/250 level titles, but since Mayo obviously does
Laver has 200 total titles to Fed's 79.
You could go through a similar analysis for Rosewall and Gonzales and see similar results, a floor level Rosewall/Gonzales above Federer. A floor level Tilden is still below Federer, but a ceiling level one is ahead so that comes to some weight adjustments which is why I have them as equals, but lean Tilden and have Laver/Gonzales/Rosewall as my clear top tier.