They probably wanted more people to participate ( but not with 50 IDs) and they want their name to go far and wide ( good advertisement for them )restricting only to their clients does not serve both.
ST
Yes, it is a publicity effort, like all such contests. Could be better planned, better organized.
By now, over the years, we have seen that initially Zerodha isn't much outside their domain i.e. trading. For broking business, they had a lot of guidance and experience of Reliance Money staff.
They have some wonderful ideas, given that, but execution wise, they need to do things over and over again to get it right, and understand that there are specialists for various activities
Reminds me of the doctors, who are very expert in their own fields. They usually are hell to work with because they think that they know better than the person doing whatever job, be it a carpenter or electrician or whatever
Doesn't hurt of hire specialists to turn your ideas into reality, and to give a very serious consideration to the specialists' guidance.