The author also says: I have copied this from the Microsoft website. Only OEMs can do this!!
And who is the author ? Just another user like you and me.
Firmware updates can be developed and provided by the OEM only upon request and with agreement from the carrier. So it is 100% up to the carrier to request this and provide the necessary technical information to be included in the carrier-specific firmware created and then distributed by the OEM.
To be honest, I do not believe that such firmware will be developed for Jio, since there is no way for Microsoft to define which handsets should get it and which not. I for example do not need or want a Jio specific firmware on my Lumia 1520 and would be extremely angry with Microsoft if they pushed it out to all handsets globally. I'm pretty sure many other users will be quite upset as well. That's why an app is required to activate it when necessary.