Never heard that Stake is offering deposit bonus since they came in this gambling industry. You can always ask their live support to make sure about the information your read anywhere outside their own website/forum. Even if it is real, it is just a common bonus and you will have to wager 40x before you are able to withdraw. However I think it is not official from stake, or I guess it is something like a special bonus for those who register with a referral link.
I think this promotion is fake as you said stake never offer deposit bonus they prefer to give free cash
Indeed, stake provide welcome offers from third parties such as streamers but there is a special code provided for every new user
Even if you want to ask support you have to tell them the promo code so they can determine if it's valid or not.