Following some of the threads on Reddit seems to add evidence for this. HBO Max is not filtering on User Agent string. Instead, it seems to be checking the Widevine keys.
Still no Widevine crack/emulation after all the Intel CPU bugs?
From my understanding, it's relatively well known how to break Widevine L3, but nobody wants to release code (for obvious reasons). Widevine L1 is much better secured, as it's required to be implemented in hardware.
This is very likely the reason, unless it was unintentional.