@Al**
the way that site puts it is misleading, it's not "actually just 29 days". It's 30 days. It's just that people count that 30 days wrong.
Put it this way: if you arrive on a Monday, and stay until Sunday, how many days is that? It's 6 nights but it's 7 days, and it is the days that are being counted. Any part of a day counts. The point is you can't just add 30 to the date you arrive, but that doesn't mean it's "officially" one thing and "actually just" another, it means that people can't count right.
This isn't a Thailand thing, every other country counts their visas the same way. It's just people don't realise how to count it. I don't blame them either it's not immediately obvious if you don't think about it and the natural thing is just to "add" the 30 days. But that would mean you would be 31 days in the country.