***** NOT A TEACHER *****
1. I am a very old man who knows nothing about the TOEIC test. But it does sound very authoritative. So I guess we have to be very careful before saying that such a prestigious test has made a mistake.
2. Would you please study these two sentences:
a. "Epeiros [a country that once occupied part of present-day Greece], as is well known, was anciently occupied ...."
b. "She did her job well, as can be proved by the record.
3. Do you think that those sentences are correct or incorrect?
4. According to the experts responsible for those sentences, they are 100% correct!
5. Let me try to explain.
a. Maybe in formal English, it is not a good idea to write such sentences, but -- nevertheless -- you will often find this kind of construction (kind of sentence) when you read English.
b. In such a sentence, "as" is being used as a relative pronoun. It refers to the idea contained in the whole main sentence.
i. In other words, it means "a fact that."
c. Now let's return to those two sentences:
i. Epeiros, as [a fact that] is well known, was ...."
ii. She did her job well, as [a fact that] can be proved by the record.
iii. As you can see, the relative pronoun "as" is the subject.
6, Now let's return to your sentence. For easier analysis, let me simplify it and change the order:
"The plane cannot leave the gate unless all passengers are seated, as was mentioned in our earlier announcement."
a. In my opinion (only), I believe that it is similar to those two sentences above:
"The plane cannot leave the gate unless all passengers are seated, as [a fact that] was mentioned in our earlier announcement."
H.W. Fowler, A Dictionary of Modern English Usage (second edition, 1965), page 38.
Random-House Webster's Unabridged Dictionary (2001), page 120.