This is correct. I sell cars here in Colorado and have been for a long time. Seller is on the hook to cover the emission test.
The best way to address this is to go an inspect the car and agree on a sale and price. Drive the car to the E-test location with the seller, or have them follow, whatever works for you both.
Verify the car passes the test before you pay. Seller should pay $25 fee. Make sure you draw up some sort of buyers order/bill of sale that you both sign and keep copies of.