It does read to me like this means 5 days of total fasting. If the 5 fasting days were spread out, then what did the participating subjects eat on non-fasting days before the "everyday food items" were reintroduced?

I don't have access to the paper and couldn't find a preprint so I cannot check beyond what is in the UPI article.

It's probably a 5:2 fast—five days of intermittent fasting and two days of 500-600 calories [1].

On the fasting days, it could be 16 hours of fasting and an 8-hour feeding window (16:8 in the fasting lingo).

[1]: https://www.healthline.com/nutrition/the-5-2-diet-guide

If they are doing intermittent fasting, then why do they have to "reintroduce" foods during the 10 day feeding window, when they've been eating food every day?

These are probably foods that weren't allowed during the eating windows.

I don't have access to the full text either, but I'd be really surprised if it's five days of total fasting. I'd expect that it's low calorie days with some special restrictions (or even meals) they've set out.