Eto is not plural. So, it would be this is my rooms, that would be incorrect.
Эти комнаты - these rooms. "This is/these are" is always это. As discussed above, "these are my rooms" would be "это - мои комнаты".
Yes, it should be accepted, and it should be the better choice.
Someone struggles with using passive in English.
AND they are trying to 'push' that edge ....
The best way to speak involves little passive tense in English.
Another mnemonic hint for portuguese speakers: remember of the word "Cômodo"
It would rather be Это мои комнаты. But I wouldn't know if your sentence is also acceptable for Эти комнаты - мои. I just feel there is a small difference to take in account, I guess.
What I can recall is that the - means "is/are" but I guess bot phrases are right
I put "theese are my rooms" and it was counted wrong even though it is the same thing.
Also pertinent is a statement by a linguist who is teaching Russian, that in Russian word order is not as important as it is in English. Yet so many of the expainations given by students here relate to using a specific word order to justify DL's answer. Either it is critical or it is not critical and if it is not then a wider range of answers may also be correct.