> I don't think it has to be ADHD.

It definitely doesn't have to be ADHD, but that loop of "new interest" -> "hyperfixation for a period of time" -> "no interest" is classic ADHD, and in almost all of the people I know who do have ADHD this was the first major sign for them.

There is definitely a difference between starting projects that don't turn out to be interesting or useful and not bothering, but the way OP described their experiences is textbook and should definitely not be minimized as it is definitely not the typical case for neurotypical people.

> I don't have ADHD, and none of my friends have it that I know of. We all start things we don't finish, though.

One of the other things I've noticed about people with ADHD or autism, even undiagnosed, is that they tend to congregate together - neurodivergent people tend to cluster together through similar interests and thought patterns. This leads to the logic of "I don't think I have ADHD because everyone is like this", but that's a selection bias that hides the distinction that people could otherwise make.

> think finishing things is a crucial skill, and we need to practice it in order to get good at it, and we won't do that if we tell ourselves it's about as good to give up as it is to keep going.

Again, this sort of phrasing can minimize the impact of the executive dysfunction inherent with ADHD; again, not saying that you or the OP definitely do have ADHD, but this idea that "finishing things is just something you need to practice at" is the ADHD equivalent of "have you tried not being sad?" For many ADHD people, it just does not, and cannot, work that way, or at least not without interventions like medication and conscious coping strategies.

> since that will end up being an excuse rather than an explanation for a lot of people

I dislike this line of thinking, because it sort of inherently (if subconsciously) implies that, without a real diagnosis from a professional (which can cost thousands of dollars that people don't have), it's probably just an excuse people are making for... what? being lazy? being incompetent? ...and that we shouldn't take those explanations seriously. If we want to destigmatize neurodivergence then we need to be willing to take people at their word and support them.

If someone says "I can't finish projects because I'm so ADHD lol", then you can either say "that's just an excuse unless a psychologist has said otherwise" or you can say "If that's the case, how can we support you, and what are you working on to support yourself?" and, if those supports don't work, then you look for alternate explanations.