Every parent was once a non-parent, and therefore can compare their own experience before and after. I'm sure parents are a little (intentionally) delusional at times, nobody wants to seem like a 'bad' or 'uncaring' parent. But to assume that the extreme majority of parents are lying about enjoying an experience that they've been evolutionarily primed for seems like flat-earth level reality-denial.
My issue is the implication that the joy of parenthood is above every other joy.
For comparison, you don't see programmers claiming the joy of programming is superior to all other jobs. Mountaineers don't claim that you haven't enjoyed life unless you climb mountains.
Why? Because those things are obviously joyful, but not accessible to everyone. Not everyone can climb, not everyone can program, but everyone can find their own joy.
So why do parents feel okay saying that? If you can come up with any other reason then please let me know because I'm still searching.
Put in different terms, there are many experiences which have an evolutionary origin. Disgust reactions to certain sights/smells are a near-universal experience, for example. Individuals that don't share these experiences (or who are attracted to them) are considered weird or degenerate precisely because they are so universal.
Sex is short-lived pleasure, dopamine. Family relationships are long-term happiness, serotonins.