This is probably legit but, for all intents and purposes, this is not a sign-up bonus. Exolix is probably not even that rich to be able to simply give away 0.005
BTC for a mere signing up without receiving anything in exchange. This is given as a bonus under their affiliate program which, of course, is a way to "give us money and we will give you a little share of it."
From the Frequently Asked Questions (FAQ) from Exolix.com:
[1]Why I haven’t received a bonus after I invited my friends to exchange?
Your balance consists of a bonus of 0,005 BTC, which you have received when registering your account and the percentage of each successful transaction that come from your account. Registrations from invited users is not enough to make a profit. It is important that these users make a successful transaction on your resource, where our API has been integrated, or through your referral link.
Highlighted portion is made so for emphasis.
So this 0.005
BTC will be given to you (1) if you are implementing Exolix's API and have people trade on your own platform, or (2) if you successfully bring traders to Exolix by way of your referral link or banner placed on your social media accounts, blog, or website.
[2]So, if you are expecting an almost free 0.005
BTC, this may not be it.
[1]
https://exolix.com/faq[2]
https://exolix.com/affiliate-program