It's actually FIFY (fixed it for you), FTFY (fixed that for you).
It's an obvious and blatant indication that the person replying made modification. It's the most up front way to make a point.
Seriously, look it up. No one thinks you wrote that when the replier does a FIFY. It's a type of commentary device on social media.
It's literally used as - "I'm going to modify your words to demonstrate how they came across to me". All you have to do is note that they replier got you completely wrong with that if you find their interpretation distasteful.
I'm not a fan of it, it's usually derogatory. But it's the same as someone just changing the quote in stealth mode. This is the opposite of stealth - it literally signals that the words are not yours anymore.
god man, I just took a sip of Dr Pepper and about laughed out loud.
Dr P is too much the nectar of heaven to waste on a computer monitor