I’d go so far as to say 5 gallons are elitist speak too if they’re not wiggling at all. There are very few absolutes in the hobby of fishkeeping. I’ve talked to a few well experienced aquarists that say 3 gallons is the bare minimum for most and even preferable for older bettas as long as you never miss maintenance. 5 gallons is just what the betta subreddit says and gets parroted off.

3 absolute bare minimum, 5 gallon recommended minimum, 10 preferred minimum (for standard tanks) is what I was taught.

E: Guess r/bettafish is leaking here. Toxic bunch (seriously, one of the most toxic subs I’ve ever been a part of) who have to have things their way even if there’s no real basis for it - they sometimes push gross misinformation. And they always move the goalpost. Don’t fall into an echo chamber y’all. The only absolute in fishkeeping is that you need water.