I'm pretty unsurprised that leveling is a good way for bias to sneak in. My experience as a man applying for a Google position and also talking to women applying for Google positions is that leveling is extremely opaque, more so than the salary offer, and the same candidate could easily move between L3/L4 or L4/L5 essentially at the whims of the recruiter and the interviewers, and the same role can be filled by multiple levels (e.g. there isn't headcount that's open at L4 but not L5). And this would be consistent with both Google's claims that people of the same role and level are paid consistently, and employees' claims of pay discrepancy.
Also, here's the original complaint: https://www.documentcloud.org/documents/4044053-Kelly-Ellis-...