For what it’s worth, I wrote a fairly lengthy post about that clause here: "without incurring any liability to IHT on my death " who? - Trusts Discussion - The Trusts Discussion Forum
Yes, I agree with your conclusion that on the first death the maximum sum that could be given to a discretionary trust without inheritance tax becoming payable does not include the RNRB because - as you correctly say - an RNRB would not be available on a gift to a discretionary trust.