Remember that there aren't as many drivers in VMS core as in Linux (were just amd DRM driver dwarfs some older kernel versions), and ACPI for all its complexity also handles as portability layer between a lot of differences that in VMS' past involved having to release an entire special release of the system just to get it to boot, now covered by ACPI support.

Also, we don't know how much of that is test code, samples (for testing, for example)