Nights spent at a Marriott will only count towards either (1) old Marriott lifetime criteria or (2) new combined SPG+Marriott criteria. For example if you're trying to make SPG Lifetime Gold, you will either need (1) 5 years of SPG Gold and 250 eligible SPG nights by 12/31/2018 (old SPG program), or (2) starting 1/1/2019, 400 Lifetime Marriott or SPG Nights + 7 Years Gold (new combined program).

Yes, the Marriott nights will count under the new program - but the requirements are much higher. So if you're currently close to SPG lifetime status and think you might hit it by end of year, don't stay at any Marriotts until you do