It's not just IPs that have reputation scores but IP ranges and AS entities. Residential IP ranges are often dinged by default because of the risk of infected home PCs sending spam.
There absolutely is a reputation associated with individual addresses and ranges of addresses. And that reputation can be derived from numerous legitimate means (honeypot activity, spam, reverse dns info), but whatever these folks are doing regarding "proxy/vpn detection" is indistinguishable from snake oil.
I would guess that all or nearly all residential IPs are marked as at least 'low proxy' risk because of the risk of malware on personal devices.