I might wager a guess:
Any year with MR/SPG elite status will count towards lifetime no matter how it was attained on a stand alone basis (BIB, credit cards, challenges, gifts, buybacks, etc).
The only status years that will not count are those that Marriott/Starwood granted in a way of reciprocal grant after the merger. And there's a reason for it: Stand alone MR/SPG status will be double counted towards lifetime eligibility (e.g. you can collect two plat lifetime eligible years in the 2018 calendar year). If reciprocal MR/SPG granted status would count as well, any status member could claim double counts, so MR would need to increase the eligibility thresholds for lifetime elite membership to account for this. The downside would be, that you'd need even more years to attain lifetime status if you have just recently joined or will join in the future. This is why MR wouldn't require more elite years for lifetime elite status.
The beauty of the aforementioned approach: counting years would be an almost straightforward exercise for MR and members.