Perhaps a more likely conclusion to draw from your "experience" is that your MySql tables were on the part of the disk that crashed and the Postgres tables were not.
No, a more likely conclusion to draw is that the MySQL tables were partly in memory at the time of the crash, while the PostgreSQL tables on disk were kept in a consistent, recoverable state.

Do you think MySQL has speed benefits only because MySQL developers are so much better than all other RDBMS developers, and not because of speed hacks that put your data at risk?

Do you think robust RDBMS design is just dumb luck?