1. What do you mean by they are impersonal?
I mean it is not mentioned who made the statement or the agreement.
2. You told me that it's a matrix clause, it was stated that, it was agreed that. What about,
It was eaten, does it mean it is a matrix clause? e.g. The burger was eaten by Donald or you can say, it was eaten [matrix clause], it does not tell you who ate the burger, and what was eaten.
For there to be a matrix clause, there has to be a subordinate or dependent clause.
In:
- It was agreed / It was stated that ....
"it was agreed / it was stated" is the matrix clause, and the "that"-clause is a subordinate or dependent clause.
In:
- It was eaten.
there are no subordinate or dependent clauses, hence the structure is a simple sentence, not a matrix clause included in a complex sentence.