... and, I'm wrong.

The link does in fact have the statement I was looking for, "When a SQL Server 2005 database is upgraded to SQL Server 2016, the database compatibility level will be changed from 90 to 100. (In SQL Server 2016, valid values for the database compatibility level are 100, 110, 120, and 130.)"

So, MS is on the record supporting compatibility with 2008 ("100") and an automatic upgrade from 2005 during the upgrade-in-place. This appears to be "safe".