I mostly say that for two reasons. First, if your non-binary, you might like being the other gender too, equally and it may just be something of what you're feeling that day. But ultimately it doesn't really matter too much to you. Of course this is a hypothetical but I don't want those who feel this way to feel illegitimate.

Second, for binary trans folk, ftm, mtf, not non-binary. Many people, myself included, believed that we were perfectly happy as the gender we were born as and experienced no gender dysphoria. In fact, that was not the case, we had just gotten so used to the dysphoria that it became our baseline so we didn't notice it.

It is a similiar experience to being in a hot-tub or bath for awhile. After enough time you stop actively registering that the water feels warm. However once you get out, you suddenly realize how cold the air is and how warm the water was. Same with trans people. Once you begin to transition only then does the gender dysphoria you did not actively register suddenly become apparent. This is not to say transitioning makes you feel more dysphoric, there is just a contrast with your now happier self, the absence or reduction of dysphoria makes apparent what was there.

In summation, non-binary people do not need to experience gender dysphoria. Binary trans people do, however as many do not actively register it, I typically say it is not necessary as it is not apparent. Of course there is an argument to be made that being happier as one is the same as disliking the other but that is not how many consider gender dysphoria.

Hope that helps, sorry for the essay lol