The T&Cs are ridiculously incomplete and/or vague and in some cases in error, especially when it comes to legacy qualification, both annual and lifetime. (They're only relatively clear on new program qualification.)

It's been said many time by the official Marriott/SPG representatives here that for the 1.6M'/2.0M lifetime points legacy Marriott qualification, the lifetime night count has to be all at legacy Marriott brands, and that for the SPG 25 stays (for annual Plat) qualification), it has to be all at legacy SPG brands. It's never been proven and never will be able to be proven until it's too late (as the nights have to be done before the end of 2018, but the proof will only come in 2019).

So you can either believe the official Marriott/SPG representatives or not. But you can't expect supporting documentation because Marriott doesn't seem interested in getting the T&Cs straightened out in the next month regarding issues that won't be around any more in 2019. And as I said, because the computation of legacy lifetime status (that wasn't done before Aug 18) won't be done until after the ability to earn legacy lifetime status is over, proof won't be possible until it's too late to be useful.