You're confusing the words 'compatible' and 'identical'. How is Level Up not compatible with 5E? I can't think of a single thing in Level Up which can't be used in a 5E game, or vice versa. Every new class, archetype, spell, rule--it's all 5E compatible. It uses the 5E rules. Unless you're saying Tasha's Cauldron isn't compatible with 5E. Or if new 5E classes makes it incompatible, then Eberron isn't compatible with 5E because it has the artificer... and D&D 2024 definitely isn't because those classes are different too.
Nah, Level Up is fully compatible with 5E. It's not identical to 5E. (it's better)