I'm Thai and I'm looking to go to South Sinai resort. A Thai cannot go to Egypt without applying for the visa first. However, according to this http://www.doyouneedvisa.com/passport/Thailand I can get visa on arrival to South Sinai resort. I'm not sure how reliable this source is? Or where can I find more information about this?
According to Timatic and Egyptian Consulate in the UK there is no visa on arrival for Thai citizens at any location including South Sinai airport (which I am guessing refers to Sharm el-Sheikh). So even if the website you linked states otherwise you should consider Timatic as an authoritative source for the airline.
Thai citizens need a visa before arrival to Egypt, that includes the special area of south Sinai. People who need visa in advance to enter Egypt are exempted from this facilitation. This is clearly stated in the official Egyptian Immigrations website, which is in Arabic only:
•لا تسري التيسيرات الخاصة بالأجانب القادمين لزيارة المنطقة المحددة عبر منافذ جنوب سيناء علي الدول التي يشترط حصولها علي تأشيرات دخول مسبقة .
So, you will have to apply for a visa in advance.