Can you expand on what you mean by a single table with manual indexes? How does a frequently used table with indexes not resemble an RDBMS?
If you use only 1 table, then it's by no means "relational", so I don't see why you'll need a database system designed from start to finish to support a relational model.
Postgres is lacking in these scenarios, whereas a particular fine tuned version (or fork) of InnoDB (or MyRocks or whatever they end up chosing) handles this better.
See Facebook's "mysql-5.6" branch, that has hundreds of patches piled on to support especially these taxing workloads.
So MVCC also has almost zero bearing on what they are doing.
MVCC and how it has been implemented in the two storage engines in discussion here absolutely has a lot to do with it.
FYI The term "relational" in "relational database" does not refer to the relation between tables but to the mathematical concept of a relation which is some set of tuples ie a single table.
But I think that's a mute point, I doubt they kept any part of the relational model inside their schemaless database.