Yes that is correct
The mysqlclient15 is simply a bridge in the sql libraries. Plesk is looking for the 5.0 libraries (the actual .so library files that its been compiled against).
If you simply try to update mysql from 5.0 to 5.1, these so.5.0 library files are removed and replaced with 5.1.so library files, hence the dependency error against anything compiled against the mysql-5.0-devel package.
This then breaks plesk as its now got missing so libraries it needs (if you were to force the update), as it can't find the 5.0.so libraries.
This is why sometimes when you remove one package, more get pulled as those share libraries themselves.
Anyway in this case the mysqlclient15 package simply is a cut down mysql-5.0 package only containing these libraries (the 5.0 ones) that are needed.
The mysql-5.1 is built having the mysqlclient15 as a dependency so this just happens nicely so it updates without issues.
So plesk is happy as it still has its libraries its compiled against.
So it will always say in the plesk gui on phpmyadmin the 5.0.90 client or say a 91 or 92 if 5.0 was continuing to be built and updated, but since 5.0 is no longer amintained it will stay at 5.0.90.
The only way to ever fix this is to recompile the actual plesk packages against a mysql-5.1-devel, but parallels will not do this as if the 5.1 is not officially released anyone not running the later mysql would have their system break.
Unfortunately its a common issue with updating packages not in the OS, you may have to add compatibility libraries, or rebuild 10 packages as these all have dependency issues.
Hope that explains it better for you.