I get from the vague wording in the UA that you are arguing that as soon as you have a spell of a certain slot (able to cast 2 2nd level spells, means 3rd level slot and thus up to 9th) that you know all. But I don't understand where you are getting these slots at first level. Perhaps it is some other part of the build I haven't seen yet.
Okay, you need four things:
- Elven Generalist. This gives you 1 extra slot of each level that you can cast.
- Domain Specialist. This gives you 1 extra slot of each level that you can cast.
- Alacritous Cogitation. This isn't actually used, but it's a prerequisite for:
- Versatile Spellcaster. This allows you to trade two slots of the same level to cast one spell of the next higher level.
(Note: I don't have the books to hand, so the above may not be 100% accurate.)
So, you're a 1st level Wizard. You combine two of your 1st level slots to cast a 2nd level spell.
Of course, this means you can now cast a 2nd level spell. Therefore, Elven Generalist and Domain Specialist each give you a 2nd level slot.
You then use Versatile Spellcaster to combine
those slots to cast a 3rd level spell. Of course, this means you can now cast a 3rd level spell. Therefore, Elven Generalist and Domain Specialist each give you a 3rd level slot.
So you use VS to combine
those slots to cast a 4th level spell...
And so it goes, all the way up to 9th level.
That's how the trick 'works'.