It's not guaranteed. I just saw a post in another group where a guy paid 6,000 Thai baht for entry but was still refused entry and had to book flight back to where he came from.
Onward tickets are still real tickets though. They are simply cancelled by the company after you enter the country. The person on Reddit also showed the onward ticket email instead of opening the PDF within the email and showing that. So this "software engineer" is just an idiot.
this has always been a requirement. They don't always ask but anyone who is smart has a ticket out of the country when they arrive. Anyone who was denied for this reason is an idiot and didn't come prepared.
expats have money. Lots of Thai people are dirt poor. The government wants money. Why would they go after dirt poor people for something they don't have?
The ask:thailand community, consisting of multiple Q/A groups with over 100,000 members, powers this platform. It is not an official government resource. Our members actively contribute to this resource, and while we strive for accuracy, we cannot guarantee its complete reliability. Assistance to travelers is provided as a community service.