I see what you’re saying; it seems as if its 2 separate entities. I sign into exchange.nash.io with email, but extension and community with username.
My assumption would be that for a good user experience, they avoided the extension and allowed signups with email account only. For the ICO, (and community login) they required a NEO address and in the case of community, proof of NEX to participate. Not something they could do with just an email.
I think they will end up connecting the two later. The extension is ok for now, but it doesn’t have an ETH address associated with it. My guess would be that once you are logged into the email account, you will be presented with creating a wallet (to add $25 to validate your account) It’s at this time it will ask for you to be logged into the extension as well and it will link the 2 and the other under lying addresses (ETH, LTC, BTC, etc)
You’ll then be able to see balances and look at your account via email login, but you wont be able to do any transactions without verification logon to the extension.