The alternate 17th green is the original one from Fowler’s design. So it “exists” but not as part of Thomas’ design.
This sort of refutes a thought I had yesterday.
The Stanford University Golf Course, which is attributed to Billy Bell, but thought to be a George Thomas routing, has a par 3 19th hole adjacent to the 18th green. The hole is steeply uphill and about 125 yards long. It's typically used as a practice area, but is put into play when a hole on the course is out of service for any reason.
I figured that's the reason the little 17th at LACC North is maintained. Occasionally you take a hole out of service, in which case you play little 17 after big 17, and then you go play 18.