Zap-King-Lu refers to passive fat boots spam team comps. These teams don't always use that exact three mon core, but the idea is the same: ignore hazards and chip down opponents with status and hazards until you can clean the endgame with your win condition (historically, this was either Zama or Gambit, but a lot of teams now use Weavile, Pult or Scor). They rely a lot on hax, with flame body, static, and thunder wave spam being employed as the primary methods to force progress.
SupaGMoney made
an excellent post (outdated now, but still accurate) during the Kyurem suspect, which provides historical context to the argument.
These teams nowadays also use Pech, Nite, Ghold, Torn-T, and other boots mons. They have a very strong matchup into both offense and fat due to hazard immunity, so the best way to defeat them is to employ strong breakers, who force progress against the defensive core and open holes for the rest of the team.
In a healthy meta, breakers will shift in and out of focus as boots spam becomes more or less popular, which we've seen happen during both SPL and WCoP. Most breakers aren't easy to fit on teams, as they usually have heavy structural constraints to make them consistent. Specs Kyurem requires hazard control to keep rocks off, Wake is weather reliant, Hoopa-U necessitates multiple pivot partners, Ursaluna is only really seen on trick room... you get the drill. Due to this, it's very easy for these comps to tech against them, for example Tera Water Lu or Glowking to destroy sun.
A lot of top players' reasoning is that if we remove enough breakers from the metagame, these teams will be able to tech to handle the remaining breakers and once again become the undisputed best team composition. As mentioned before, the heavy reliance on paralysis and pivot spam makes them extremely infuriating to face, so a return to that metagame would be extremely undesirable.
The last three mons to be suspected (minus Palafin) were all breakers: Moon, Gliscor, and Kyurem. Moon in particular was one of the few splashable 'mons that was able to force progress due to its strong STAB knock; you can draw a direct line from its ban to the recent outbreak of veil and webs in tours and ladder, as the removal of one of the only consistent HO breakers drove players towards cheese.
As to whether or not that argument is valid: it's up to you. I personally agree with it, but see why others might not.