Cat8 is an existing standard (so they could be real, only a real cable tester could confirm the required bandwidth), but you could’ve saved the money and bought shielded Cat6(a) cables. Unless you need to go 50m+ or are in an RF heavy environment, even using Cat7 has no benefit. Unless you put those cables into a wall and can’t easily swap them, using Cat8 is absolute overkill.
Heck, even shielded Cat5e works with 10GbE! But only shorter runs with a high quality cable.
He probably isnt getting datacenter grade cables though. Sounds like he bought a bunch of cheapo ones off Amazon or something and these are likely not even fully cat8 compliant as many such cheap cables have that problem. Ive seen plenty of "cat8’ and “cat7” cables that perform worse than a reputable brand cat6.
Good quality CAT6 and CAT6a cables are actually not that cheap so if you bought it on Amazon or something it’s most likely not CAT8 and/or not pure copper.
10GbE is within spec for CAT6 for short runs and CAT6a for runs up to 100 meters. I would recommend getting your cables from reputable distributors of networking equipment, even if they at times command a premium.