It’s not about incoming cash - It’s about generating more investments in Thailand rather than foreign.
So first they created tax ruling
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to impose better taxation on results from foreign investments. Then they figured the ruling had an adverse effect because investors would simply keep their money abroad rather than investing it in Thailand.
So now they relax the same ruling, but only for foreign investments, so it seems. Unfortunately leaving (some) expats like us in the middle.
Actually, legalization of documents does occur in the home country.
I can’t speak for USA, but in Netherlands you first go to a legalization office of the government (Ministry of Foreign Affairs) who validate the document and the stamps/signatures on it and then go to the Thai embassy in USA to validate the signature of Foreign Affairs.
It works exactly the same the other way, when you want to validate an official Thai document for use in your home country.
So, in short, yes you need to visit the Thai embassy in your home country.