You can comment from the old interface just fine. I use the old interface without going to old.reddit.com - there's a toggle in the settings to stick to the old

You can comment from the old interface just fine. I use the old interface without going to old.reddit.com - there's a toggle in the settings to stick to the old design. The only time you need to use the old.reddit URL is when you aren't logged in but then you can't comment anyway.