Welcome to the forum, drs8!
“..when this attachment to the body lessens to a degree, you become Sotapanna. And when this attachment lessens more and that is Sakrudagami. And once the whole attachment to the body completely gone, this person becomes Anagami.”
Of course this is wrong.
Even to attain the Sotapanna stage, one needs to remove not only sakkaya ditthi, but also, vicikicca and silabbata paramasa; see, “Sakkāya Ditthi is Personality (Me) View?“.
Furthermore, contemplation of the 32 body parts only helps partially in removing sakkaya ditthi. Sakkaya or “sath” + “kaya” means one takes not only one’s body to be “good” but also one’s actions in accumulating things in this world to be “good” and “beneficial”. Kaya can mean body and actions.
We can also see this clearly in the Satipatthana sutta. Kāyānupassanā is only one of four anupassana, and contemplation of the 32 body parts is only part of Kāyānupassanā; see “Maha Satipatthana Sutta“.
One becomes a Sotapanna by comprehending the fruitlessness of doing “apayagami immoral things” to gain sense pleasures. Then one has reduced kamaccanda, vyapada, and moha to kama raga, patigha, and avijja levels.
One becomes a Sakadagami by attenuating kama raga (specifically removing vatthu kama) and patigha. Then one becomes an Anagami by removing all kama raga and patigha. In both stages, avijja is also reduced.
Finally, one becomes an Arahant by removing the higher five samyojana of rupa raga , arupa raga, mana, uddacca, and avijja; see, “Dasa Samyōjana – Bonds in Rebirth Process“.
So, it is easy to see that one cannot even become a Sotapanna by just doing the contemplation the 32 body parts.
I am not familiar with the Thai Forest tradition or Ven Ajahn Mun’s biography. Are you certain that this is all they do (contemplation the 32 body parts)?