I don't think it's actually a requirement, although I'd say it probably should be? As I'm not aware of any tourist travel insurance which would come close to covering that amount of time.
To the op, I'd certainly want a very large stash put away to cover potential medical costs,, could be hundreds and hundreds of thousends?
What no one is telling you and where your confusion is,,
The airlines have a legal responsibility for you to arrive in any country with the correct paperwork and they are legally required to return you, from where you came from, if that paperwork isn't correct on arrival.
When you fly in on a visa exemption to anywhere, you don't have an agreement with that country. The country are simply allowing you short access free. So in this situation the airline need proof of an onward ticket within whatever the free allowance might be.
If you have a visa, technically you have an agreement or contract with the country before departure.
It's labelled correctly, for their target market, most wont want residency.
If you have a working visa, or a retirement visa, it doesn't make you a national, or necessarily a resident to the Thia government. But your own country may well class you as a resident.
Nomad, digital worker, remote worker, bla bla bla.
Call it what you like. It's basically a "have you got enough money visa" to stay here , isnt it?
As I said, the 180 days is likely about tax and 99.99 percent of people, or dtv holders will draw a greater amount of money from another country. So the tourist label would suit those who it's aimed at.
Id say, even the retired (uk) might not have their pension payments frozen under these guidelines?