5 fasting days doesn't mean 5 days of total fasting.
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.
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