Xtol is far from "so sensitive" to Oxygen, but not following the mixing directions on the label you shouldn't be too surprised that you got thin negs. Mixing with regular tap water at 20C and pouring the whole bag into a jug and shaking it 'like mad' until dissolved is just plain Wrong and Incorrect. BAD. Don't do it again. If you want to use Xtol learn how to mix it, its not D-76 or Rodinal, there is lots of proper guidance on APUG.
I've used Xtol for more than 10 years, for unrepeatable wedding photography so I value highly consistency and reliability.