Subsection 3.3.2 (Timing of Debit Entries):
An RDFI must not debit the amount of any Entry to a Receiver's account prior to the Settlement Date of the Entry, even if the Effective Entry Date (as defined on Appendix Three (ACH Record Format Specifications)) of the Entry is different from the Settlement Date of the Entry.
I can't tell you whether NACHA would consider the memo post a violation of subsection 3.3.2.
Your counter to the audit report could be that the bank didn't actually post the entry early. It placed a hold (via the memo posting) on the funds because it would be paying them out (via the Settlement) on Tuesday, the Settlement Date.
The argument on the other side of this issue is that, if other items brought the account negative on Tuesday, the 15th, you could make a timely return of the ACH debit to reduce the amount of the overdraft or eliminate it, yet the memo hold effectively denied the Receiver, your customer, the use of those funds over the long weekend.