The persistence allows previous states of the data structure to be used in still ongoing, "older" transactions.
The term "MGA" also seems to be a rebranding private to the Interbase universe so I question that "MVCC aka MGA" bit as well.
"Although InterBase's implementation is much more similar to the system described by Reed in his MIT dissertation than any other database that existed at the time and Starkey knew Bernstein from his previous position at the Computer Corporation of America and later at DEC, Starkey has stated that he arrived at the idea of multiversion concurrency control independently. In the same comment, Starkey says:
The inspiration for multi-generational concurrency control was a database system done by Prime that supported page level snapshots. The intention of the feature was to give a reader a consistent view of the database without blocking writers. The idea intrigued me as a very useful characteristic of a database system."
Seems you're right.
Curiously, I'd assumed before starting to read about MVCC that history was done at a page level as described above, whereas it seems it's generally done at the row/tuple level