The feedback the account has received is 1-2 years old. I guess they wouldn't be in the market anymore if they were really a scam service.
Of course they could be, and they will continue to be in the market as long as people ignore their negative feedback and continue to send them money.
Go and look at the accounts of some of the scam mixers, or scam exchanges, or gift card scammers, or scam ICOs, or game-protect, etc. Filled with negative ratings from years ago, and yet people continue to lose money to these scammers because they either don't look at their reviews first, or ignore the reviews for reasons known only to them (including, I suspect, for the exact reason you have stated - they are historic).
This isn't just a single incident or bad review from a single disgruntled customer against Changelly. There are a ton of bad reviews on here and reddit. Not to mention they give you truly awful exchanges rates.
I would be avoiding them.