Also known as soft realtime.
For hard realtime, you need guarantees. Linux is too complex and cannot possibly offer them; this is the territory of formal proof, with complexity growing exponentially with code size.
Look at seL4 for hard realtime.
Fortunately, for realtime audio, you don't need hard realtime unless you're very close to overloading the CPUs. Soft realtime (i.e. 99% of all interrupts handled within N usecs, 100% handled with N*2) is good enough.
AFAIK Linux RT kernel is designed to hit every deadline, but they just cannot offer a formal proof.
So "more-or-less" hard real time.
Usually when you're entering that realm of such requirements, you're rarely talking about off-the-shelf devices.
And that's why it is soft realtime. No guarantee, no hard realtime. It really is that simple.
Linux is still Linux, and it is still fundamentally not capable of hard realtime.