While his wounds were reversed in his Bankai, so were his enemies, meaning that any attack he performed was rendered moot by his own reiatsu.

Source: https://www.cbr.com/bleach-cant-fear-your-own-world-light-novel-shuhei-hisagi-bankai-represents-tosen-ideology/

Yhwach's power extends to absorbing and dispersing reiatsu, adding to his already enormous might.

Source: https://gamerant.com/bleach-most-powerful-characters-in-the-quincy-blood-war/