Resurrecting this topic as I've run into some issues with missing credit.
M&M ran this promotion in September and I booked a stay - I don't recall the exact rates but it was either 6x or 8x, plus optionally 2x for Bonvoy members.

The stay was in late Nov, and after it didn't post I raised a retro claim request. M&M got back to me saying that Marriott rejected the claim since I already earned Marriott points for the stay, and referred me to Marriott in case I have further questions. I pushed back (two emails!) to M&M saying that the promo is on top of Marriott points per the terms (luckily current promo has the same terms!) and reiterating that this was M&M promo that I expect them to honor. The second reply was very firm to say "you already got Marriott points, you can't get both".

Any advice on how to take this further, as I'm quite keen to get the ~4-5k points I'm owed?

FWIW I did all the right steps - clicked on M&M link in a fresh incognito window, booked on Marriott.com, and completed my stay.