Tesla does not guarantee anyone they will get a rebate/refund. Just mentions that at this time they are available, but you should check on any details that will make you perhaps not eligible. Could be that the govt. changes their policy, that your income make your ineligible, or that your specific vehicle will not be delivered in time or may not qualify due to price etc.
Some cars will be delivered in time, others will not meet the recently changed deadline.
In the past, Tesla has guranteed that people ordering by a certain date would receive a refund from Tesla if their cars were not delivered in time. Those people delivered after the EOY, but before a certain deadline, did indeed get checks from Tesla. This was an incentive to get as many orders in as possible for EOY deliveries.
As far as I know Tesla has not offered that incentive again this year. Sales have been strong and they are delivering all they can make. Hard to justify a refund in this specific case...but you never know.
Elon is well aware of the mass of people wanting their cars delivered by Dec 3 and EOY, Imagine that they will be working all hands on deck as we get closer to those deadlines.