The pertinent ruling is apparently
608.2g: If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a-i, except no player receives priority after it’s cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.
EDIT: the above rule is why you can cast spells with Gix. There’s a ruling on Xanathar that says “Xanathar doesn’t change when you can cast spells […] You must still follow all normal timing rules”
With Gix, the ability lets you cast the spells as part of its resolution, as the ruling you shared notes. It’s basically saying “Exile the cards, and put any number of them onto the stack.” You don’t get the ability to do this at a later time, only right as the ability is resolving. Since that would preclude casting anything but instants, it has to bypass timing rules by necessity to function as intended.
Xanathar doesn’t do this - it gives you an ability that you didn’t previously have (playing cards from the top of your opponent’s deck) until the end of the current turn. The pertinent wording that is absent from Gix and tells you which type of effect you’re dealing with is the ‘Until end of turn, you may…’
Ahh, so in the rule it says “If an effect specifically instructs or allows a player to cast a spell during resolution” - Xanathar is instructing the player to cast a spell until end of turn, rather than specifically during the resolution of the triggered ability in the upkeep where you choose your target opponent. Makes sense now, thank you
Exactly, yep!