It is not, in fact, a legal routing for any current fare filings. It may have been in the past, but it is not true now. See routing rules below from EF. Given that it's not a legal routing, the only way to book it on a single ticket is with a broken fare. But DL won't let you book broken fares through hubs where they do not have a fare filing unless you have a connection longer than 4 hours to create a true fare break.
MAP CONSTRUCTED LEFT TO RIGHT AND RIGHT TO LEFT
1. GRR-MSP/DTT-ALB