Using this tax calculator https://www.uobam.co.th/en/tax-calculation, with 5 M income and I use all the possible deductions/allowances - wife, 3 children, 4 parents, 1 disabled person, all the possible investments, life insurance etc. I come to a net income of 2,683,819.95 and tax to be paid 570,145.98, which is 11.4%.
A tax adviser creative more than me can easily come to 10%.
Back to the Bangkok Post article. A few quotes that's interesting:
"For example, salaried employees bear 80% of the total personal income tax burden. However, only 4 million individuals pay personal income tax, though the labour force tallies 40 million. This means only 10% of the total workforce pays income tax."