The other person’s average is a bit, odd.
When you say they “did some intervals but not hitting lap button” how is it then that you say their ride had Strava auto-lap? I’ve never heard of Strava 5 mile auto laps before. Only for running have I seen Strava auto laps.
The 7w isn’t surprising, my Trainingpeaks avg power is always a couple watts less than Strava. I’m guessing how it computes the stop/starts and otherwise is different.
If you’re ok with it, screenshots or links would help.
Personally, I setup a LOT of private segments for purposes of making sure the data/speed/aero seem right for a workout. Even if it’s a one-time visit, after the ride I pick some obvious spots and then create them.
I almost use the Strava private segments as permanent auto-laps, without being bothered mid-ride by a ton of auto-lap notifications on my gps.