This is good news, though I suspect it is more of a business decision rather than anonymity driven. If they have a Tor service, then it will enable users that wish to trade in areas where crypto trading is forbidden the opportunity to trade with confidence that no one is eves dropping.

If you use Tor and navigate to a public address (bitfinex.com) then who ever controls the Tor exit node may have the opportunity to interfere with your traffic. You can be 100% sure traffic is not leaving the Tor network when using an onion service. In this case, it will increase the security of your trades.