[nn3]

A lot of modern databases don't use Btrees anymore. With a LSM structure you can use other suitable data structures because they can be write-only. For example leveldb uses a simple binary search.

[tannhaeuser]

https://symas.com/lmdb/ begs to differ

[craftyguy]

They didn't say 'all', they said 'many'. So your one example doesn't 'beg to differ' much in the way of OP's point.