it used to count with SPG, but two years have not passed since Amex started transferring directly to Marriott accounts (this started in August 2018), so nobody will have any 100% conclusive data. However, it’s obviously a very good sign that the expiry date moved back in your account, and a line in the terms and conditions is definitely in your favour: “converting Points to Miles or Miles to Points does count toward maintaining an active status;”. It’s a little ambiguous since an Amex MR point isn’t exactly a “mile”, but I think the spirit of the rule includes all transfers in from external loyalty programs, including Amex transfers.

If you are worried about some IT sweep disallowing the transfer as an eligible transaction and changing your expiry date, I would screenshot your account now showing the longer expiry and use if you ever had any trouble. But I think it’s a very low risk.