I guess people just come up with that being the “real” thoughts is because a person wouldn’t be saying that if they didn’t have it in mind in the first place.
A person’s real thoughts on you are often expressed when distance is large and your time doesn’t match anymore. Usually because they get the courage to do so after it’s sure that you won’t be seeing or hearing from each other anytime soon or at all.
But of course, we have to consider the fact that humans are commonly driven by emotions and what they say aren’t usually what or how they meant it. So saying that those thoughts are “real” would be unfair.