The clause might be stronger, but there’s no EU forces, no EU equipment, no EU AWACS, and no EU command structure, to back up that clause. There’s many individual national militaries, but no dedicated EU military. NATO on the other hand has dedicated forces, equipment, command structure and so on. Logistics wins or loses wars. So even if the clause is stronger, is carries much less weight than NATO.

Being in the EU is however a decent deterrent for most purposes, but maybe not sufficient to deter Russia, China or USA.