The stamp indicates the admitted length of stay (here 90 days) based on the single entry Non-O visa. The red stamp itself is NOT a visa; the visa itself has changed its status to USED and is not valid anymore. By the way, I got exactly the same red admission stamp and handwritten remark but with 1 day earlier because I entered Thailand on 11 July.
I arranged many German visa invitations successfully and none of the Thai visitors were "employed", no employment contract, no holiday letter, ... They lived in a rural area with the typical agriculture-related jobs. And some of them are elder--already in pension receiving age.
Strange. My local DLT extended my Thai Driving Licence having either a Tourist Visa or Non-Immigrant O Visa (NOT the extension based on that) each time without problems.
Concerning Thailand, tax residency is defined solely by the total number of days a visitor is staying in Thailand per year. If living less than 180 days, he/she is not a Thai tax resident.