Xyston-class are quite a bit larger than Imperial-I-class, so theoretically, they could hold more. Except they can't. According to Wookieepedia, an Imperial-I-class can hold 144 TIE fighters of various types, plus many walkers, a few shuttles, and some troop transports. A Xyston-class can only hold 72 TIE/dg fighters The setup for a superlaser with similar power to the ones found on the DS-I and DS-II Battle Stations takes up quite a bit of space. That is space that can't be used for TIE fighters. Also, against a 14,000+ strong fleet of larger ships (https://starwars.fandom.com/wiki/Battle_of_Exegol), 72,000 TIE/dg fighters, with deflector shield and 4 laser cannons (2 L-s9.6 and 2 L-7.0) (https://starwars.fandom.com/wiki/TIE/dg_starfighter) probably wouldn't have been able to do much, as the Millennium Falcon alone has been known to take down at least 4 TIE fighters without getting shot down, as seen in A New Hope. It probably took down more in the Battle of Endor. A quick calculation based on 4-5 kills per ship gives us a figure of more than 63000 TIE/dg fighters without taking down any of Lando Calrissian's fleet. This doesn't even count the Resistance Navy, which had trained fighter pilots, so they would probably have higher counts. The 72,000 TIE/dg fighters could have been taken at longer range than their cannons could hit from the capital ships that Lando brought as well, likely increasing the kill count even more, probably well into the 6-figure range total. So Darth Sidious was probably holding those in reserve for his galactic conquest, especially seeing as he could use Force lightning to incapacitate the incoming ships. It seems to have been a show of contempt. In addition, the Sith Eternal military (name conjectural, see https://starwars.fandom.com/wiki/Sith_Eternal_military), a branch of Sith cultists (https://starwars.fandom.com/wiki/Sith_cultist), were Palpatine's military force. According to Wookieepedia, Kylo Ren slaughtered most of the cult on Mustafar to assert dominance while looking for the source of a broadcast of Palpatine's voice. This implies that they may have been low on numbers which would make it hard to staff the 77760+ TIE/dg fighters on the 1080+ Star Destroyers.