My website contains below 20 URLs between pages/posts and the sitemap memory is 6.7 MB
While his website has more than 100 URLs and total sitemap memory is 2MB
As I said before there is a magic trick, I think only someone whit good expertise can do this.
+ his sitemap Request ID can be changed between 40 minutes
While my Request ID changes every refresh.
screenshots for my sitemap: https://prnt.sc/nbj68d
competitor: https://prnt.sc/nbj68s
Conclusion:
I think there is a magic trick because of the computer sitemap page looks blank until view source page we are using a same plugin XML sitemap generator.