Just want to share a dp, recently flew BKK-SIN.
Bought WE tickets for significantly cheaper price (by about $200usd compared to buying direct with TG), flight however was operated by TG.

Loaded my *A FFN through OZ, and I got my extra 20kg baggage, and lounge access to TG lounge for me and my flying companion.
Like many have stated, as long as the "operated by" flight is by TG, and check in at TG, all the alliance status applies, even when buying cheaper tickets marketed by Thai Smiles.