The MIT/LMI/TI Lisp Machines were not, they ran a simple RTOS written in "assembler" that checked for interrupts from devices and woke up Lisp threads to handle

The MIT/LMI/TI Lisp Machines were not, they ran a simple RTOS written in "assembler" that checked for interrupts from devices and woke up Lisp threads to handle the event. The assembler code also interpreted the instruction set of the VM. Someone could write a modern lisp that sat on top of a Xen unikernel that would work in a similar way, the compiler could generate native instructions though. I think you would need to examine the Symbolics microcode carefully to categorize what it was doing.