A lot of realtime systems don’t have sufficient resources to run Linux. Their hardware is much less powerful than Linux requires.

Even if a system can run (RT-)Linux, it doesn’t mean it’s suitable for real-time. Hardware for real-time projects needs much lower interrupt latency than a lot of hardware provides. Preemption isn’t the only thing necessary to support real-time requirements.

realtime just means execution time is bounded. It doesn't necessarily mean the latency is low. Though, in this sense RT-linux should probably be mostly thought of as low-latency linux, and the improvement in realtime guarantees is mostly in reducing the amount of things that can cause you to miss a deadline as opposed to allowing you to guarantee any particular deadline, even a long one.

Sure but that was already mentioned before the comment I was replying to. Standard hardware not being great for realtime has nothing to do with hypothetical realtime Linux.

what kind of Hardware is considered to have "lower interrupt latency"? Is there some kind of Arduino board I could get that fit those lower interrupt latency required for real-time but still support things like Bluetooth?

Take a look at the Cortex R series. The Cortex M series still has lower interrupt latency than the A series, but lower processing power. I imagine for something like Bluetooth that an M is more than sufficient.