She had cooldown reductions. She received no innate increase to the damage of each spell.
Change that "some of" to a "most if not all of".
Illaoi is like heimerdinger or urgot. They punish you for playing poorly or making stupid decisions. If you don't do that they become a fish out of water because they lack the tools to effectively initiate on their own, especially post lane phase.
Illaoi is a textbook match up of avoid 1 spell, farm until you're so fat she barely tickles you. She has little she can do to surprise you, so you simply need to know what she does, how she wants to do it and how you should react.
It is tougher, because juggernauts in general shit on tanks. If you picked maokai after she locked illaoi than that was your first mistake if you weren't confident in surviving her.
Perhaps take second wind instead of bone plating as her damage is mainly spells. And I would recommend taking domination for the additional healing.
Maokai gains allot of his survivability through his passive and heals so boosting that will also boost your sustain. Given their team comp, going chain vest into spirit visage would've been fine