"room type Guarantee" is a bit of a misnomer. They guarantee actually "bed type". From MR T&C:

Guaranteed Room Type: Guaranteed priority for Your requested type of bed. Subject to availability at participating locations only for North America when your qualifying Elite level and specific preference are noted on the reservation for your stay. (Pillow, room location, extra bed, crib requests, etc. are not included in Guarantee.) Outside U.S. and Canada: smoking/non-smoking preferences may also be given priority, based on availability; exact bed sizes may vary.

So if you reserved 2 beds, and they gave you 2 rooms each with a king bed, then they have satisfied this requirement.

At check out you should have ensured they had all the charges posted on maximum of 3 folios to ensure your points post correctly as required under the T&C as well:

A Member is eligible to receive Points/Miles for staying at participating Marriott brand hotels or Ritz-Carlton hotels for his/her room and up to two additional rooms. At least one of the rooms must be reserved and registered in the Member's name and the Member must pay for all rooms, which payment arrangement must be requested at time of hotel check-in. The Member must also stay in one of the rooms. To ensure Points are automatically posted to the correct Membership Account, the Member's Rewards Program Membership Number must be present on his/her room folio and up to two additional rooms, as applicable.


But from a customer service standpoint, I agree they should be accommodating you after the fact in this special situation.