If you watch his video about it he shows some snippets of the contract he has with his MCN. They are intended to paint his case in the best light possible but even then it's ambiguous as to if the membership money is 100% his or not.
They clearly outline he gets 100% of "Google Net Revenue":
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They go on to specify what that actually means:
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The main area in contention is the Google Subscription part. I imagine the way the MCN reads this is that "Google Subscriptions" refers to the revenue they get from people who watch that used youtube red (now youtube premium) and not from people who become members, that's why they include the "for the avoidance of doubt". For reference this contract is from 2017 I believe so this predates channel membership as a feature on youtube.
They then cite a3 as justification for taking 30% under "Other Net Revenue":
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To me it just seems like he was too lazy to renegotiate to explicitly state where membership money went when memberships came out and it's vague enough for them to read it either way. Other creators with the same MCN are complaining about the same split on twitter so I imagine this is just how the MCN sees things and not a target at Ethan.