Yes it correct if youre talking about your SALARY. It is thai sourced income because income is sourced where it generated from not where it is paid from (and this is a trivial concept in accounting) . You can ask any accountant or consult any global accountancy firm or their online info, eg pwc tax summaries, and also thai rev depts info and it all says this in black and white. Of course the amount you pay is net of what you pay elsewhere withstanding DTA agreements. And by the way... this applies whether youre a tax resident in thailand or not, ie from day 1 that you start working here. Heres some proof
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Foreign source income is income like pension income from overseas, dividends from companies overseas, bond income from US bonds, rental income from properties overseas etc , because the income is generated by an asset overseas, not an source in thailand (eg you and your work performed)
The reality is 99% of 'digital nomads' including the ones in here just dont have a clue what theyre talking about.... so be careful and get professional advice if youre still unsure.