What do they mean by '99% correlation with DEXA'?
Have you looked at the graph on the Withings site...?
https://wcs.withings.com/fr/fr/blog/recherche/body-scan-dexa-scan-correlation
When a user sees '99% correlation' they are likely to assume that means if your dexa says 15% body fat, the Withings is going to be that +/- 1%, right?
But look at the graph here, it says that green area is +/- 4kg, and you can clearly see there are readings on the edges or even outside that, meaning they are more than 4kg out, even down at the 10kg area, which means at that point it was more than 40% out!!!
It may well track broadly in the same direction, but when the fat mass is showing that much discrepancy it is very misleading to claim 99% correlation.
[Edit - additional thoughts]
Looking at tis critically, it seems to only be saying that 99% of the time it's within +/- 4kg - which is a HUGE variance compared to DEXA.
For example: at my last DEXA I was 81kg, 13.1kg fat (16.1%).
If we allowed the +/- 4kg from Withings, the body scan results could have been between 9 and 17kg of body fat, which would have been a range of 11.2% - 21% body fat, i.e. absolute garbage results in real terms, but still within their apparently 99% correlation.
I'd really like someone from Withings to comment on this and set us straight, as their own promotional material definitely doesn't stand up to a little scrutiny here.