said is far too generic, but the first part is describing the general rules without mentioning DTA which has impact. So in my opinion it is not BS, but just incomplete. To be complete is almost impossible because every DTA is different.
Do you really think Thai tax office believe a farang lives from 9000 baht a month? Even the average Thai uses more. There is a reason they want non-o retirement have income of 800K a year. So when someone claims to spend less than half of this, it looks suspicious.
Yes, money earned in 2025 and brought into Thailand in 2027 is taxable. If you have to pay tax depends on the situation and DTA. Why do you think it should not be taxable? I t is up to you to prove you already had this money before 2024.
The only "rule" wirh 3 months is that applications with an INTENDED date of arrival are cancelled with no refund by most embassies. Intentions can change with or without reasons. The only thing I can think about is that in case of soft power you might get questions why you do not following the training, but even that is highly unlikely.