Since you already have green to cast these spells, fetching Izzet Guildgate and Orzhov Guildgate means you have access to all 5 colors
Their status as a Tuner also introduced Synchros to the deck, even if the one introduced to the archetype with this deck rarely sees play in modern variants
A: I think its a mix of everything
If I did have to pick a recent set to compare it to, Id probably say Lost Caverns of Ixalan , with the tendency for aggro decks to pivot towards indirect damage in the lategame, and the powerful lategame in various colors and flavors
The Beast, Deathless Prince rejected Lorcana card, Im sure A threaten Commander that isnt Brion Stoutarm
In organised tournament play this rule is taken a bit further, and you're only allowed one in any deck you submit