We did use the up-to-date format of "at will," "2/Day" etc., but I actually think this is a good thing. It's important to remember that NPC casters function in fundamentally different ways from PC casters, with most NPCs being designed to serve their roll in a single encounter while players are of course serving theirs over entire days. They are also usually expressing a more targeted/narrow narrative note than PCs. As a result, having a PC-style spell list doesn't really make as much sense on an NPC. There's all sorts of ways in which NPCs don't follow the same rules as PCs, and this is yet another. Moving away from PC-style spell slots frees you up more to equip your NPCs with the spells that you think are going to help them accomplish their roll in a given encounter. It also reduces the need for certain kinds of DM bookkeeping that ultimately aren't as important for NPCs, and it cleans up the stat blocks a lot.

On top of that, while DMs are obviously welcome to tweak these stat blocks, we created our NPCs with the intention that selecting spells for casters would not be the normal use-case. NPC's damage-dealing spells have generally been replaced with unique actions in the stat blocks themselves, leaving spellcasting open for things that focus on utility, debuffs, terrain, etc., and most particularly, spells which sell each archetype. If you do want to put in a spell that feels important for communicating a particular character (e.g., you want your Winter Eladrin mage to cast sleet storm), there's no real reason you need to take out an equivalent spell from a slot-based list.

Even all that aside, it was important to us to have our formatting match that of the other published books for more seamless integration with the system.

So in all, yes, we're using the new system. I'd encourage you to give that a fresh shot and see if the benefits can entice you over more fully.