It doesn't necessarily mean assembly, just the absence of a separate operating system. It looks like Ultibo is mostly written in Pascal, with a small amount of inline assembly to initialize the hardware, etc.
I think the line there is blurry. If someone said "bare metal Smalltalk" I'd probably assume they were referring to a Smalltalk VM running directly on the hardware.
Bare metal Java exists, because Java-oriented CPUs were and are a thing.
Have you realized that every smart card you have, like a bank card, or a SIM card in your phone, runs Java code inside, using such a Java-optimized CPU? https://en.wikipedia.org/wiki/Java_Card
https://github.com/ultibohub/Core/blob/master/source/rtl/ult...