It's an offer. Not a glitch. It's still available for Fiber for me. Based on recent experience. Not committing 3 months with them. Even it's half the price we are paying.
In case you guys don't get the 3 + 3 months as claimed on MyJio app, you can lodge complaint on consumer grievance forum (PG Portal) along with the evidence (screenshots showing the offer) and Jio should obliged to fulfill it.
Update: Jio team called me yesterday and they said it was technical glitch and can't provide the said offer. They offered me for a refund to original payment source, which I denied and it'll remain in my balance.