"...only hot data..."
Translates to "don't keep a complete backup on site, buy it from us."...dirty. Or you can look at it like you have 1.5 backups... dirty. Or, even if this was free, you don't have a backup period unless this company has it... dirty. Just... dirty.
At that point I'd just get a cheap web host and copy data X days old to it. Of course, why you wouldn't just increase your NAS size is beyond me. This company sounds like it's preying off the ignorant with FUD, but that apparently works well for the bean counters.
This service, while not identical, reminds me of the time I thought I could get away with only backing up modified files to a floppy. Yes, I thought this once, at least until the non-modified files were expunged to make room for more modified ones. Guess how that turned out. 1.5 backups actually equals .5 backups.