I'm afraid that's not the right calculation.
Price for Tv = 1.39L.
with HDFC No cost emi option for 24 months, the price for tv is still 1.39L, but you will be paying 1.39L/24 = 5791 each month, for 24 months.
Now, how are they achieving this?
They calculate it in such a way that InterestOn( X amount) + X = 1.39L.
When you solve for X with the interest %, you will arrive at X = 1.2L, which implies a 19k discount, which is what Amazon will charge your credit card.
There is nothing to credit here, they will charge you the discounted amount in the first place.
Also, your second line
This would mean that HDFC gave you a bank loan of 1.39L without any interest, which is illegal according to RBI.
The discount here is provided by Amazon and not HDFC.
If thats how it is working , we can try it out but my best guess is the EMI is the tricky part.
I am just telling my current scenario [may be it is not applicable here]
for me 1.25L for 12 months with non Cost EMI option.
My Monthly EMI is : 11282 * 12 = 135384
and for me cashback is : 10387
So total effective price is : 1.25
So , if i want to close the loan to bank i am liable to make 135384 , not 1.25L.
And in your screenshot it says clearly [ The bank will continue to charge Intrest on EMI ]
I may be completly wrong here , but it is a worth to try this option