I just did a git pull on Anarki and indeed now (len nil) returns 0 as it should. However, (len 'sym) still returns 1, and I have to ask why this is so and not an error the way all other non-nil/string atomic values are.
I don't like len!sym being 1 either. I think ppr.arc is the thing that should be fixed, perhaps by having it use its own spinoff of 'len. But I didn't try to fix that; I just did the minimal change needed to get len.nil to work properly again.