It also helps that the creator of the default storage engine (WiredTiger) are Keith Bostic of BerkeleyDB fame [0] and Michael Cahill, whose PhD thesis on serializable snapshot isolation [1] formed the basis of Postgres concurrency control [2]. Notably, both of them still work for MongoDB.

[0] https://en.wikipedia.org/wiki/Keith_Bostic

[1] https://dl.acm.org/doi/10.1145/1376616.1376690

[2] https://wiki.postgresql.org/wiki/SSI