Sounds like Siam Legal doesn't have a clue. There is no "5 month tourist visa". There is a "Multiple Entry Tourist Visa" which can be obtained in a persons home country IF it has a Thai embassy or consulate, i.e. not available for everyone.
That right there says "get a tourist visa in advance", which will also smooth over any problems with airline check-in. Thai immigration on arrival is not usually a problem.
It's not a "visa" it's a border pass, which costs 500b (or $10 if you're lucky) and limits your travel to Myawaddy. A real Myanmar (e)visa can otherwise be done online @