> If you squint, any database engine is “in memory” if there is more buffer than data.

That is sadly not true, I remember one lonely night debugging a MSSQL 2012 instance that was _very_ slow, and it turned out that for a simple query (one join, 100 rows in one table and 10 in the other, 100 result in total, one where clause) it forced writing the result to disk before evaluating the WHERE condition. Unable to fight the scheduler I've ended up making a ramdisk for this data.

Yes, tempdb spills can be annoying, but I believe they never occur for queries that use only in-memory tables.