The "relationalness" is/was not really important here. It's all about MVCC and how storage engines handle it.
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.