There are various English words to translate Thai words and it is not for me to say that one word is correct and the others are not.
Taking the example of the "actual legal definition of the law in Section 38 of the Immigration Act of 1979", for which you kindly Quoted the source as https://herorealtor.com/simple-guide-to-tm30-thailand-immigration-requirements/, I called the quoted text an unusual translation because I have rarely seen "landholders" and have never seen "leaseholders" in connection with a definition of Section 38 of the Immigration Act.
Admittedly, the quoted text is merely the website owner's personal definition of the meaning of Section 38 and does not pretend to be a translation of the Thai text of that section, but it serves as an example of the confusion this can create when one considers that the Thai term for which "landlords" appears to have been used here is variously translated elsewhere as house-master, housemaster, householder, owner, lessor, and perhaps a few more things.