EyeDiscomfortCertificate No there is no "only two panel manufacturers" and Samsung does not produce any panels since 2018(?), however the do design their panels and outsource their manufacture to CSOT and AUO. Now there 4 major IPS panel manufacturers (BOE - by far the market leader these days, LG, Innolux, AUO) and 3 major IPS panel manufacturers (Innolux, AUO, CSOT). Neither LG nor Samsung put their panels in their monitors- LG uses exclusively BOE and Samsung does not make panels at all.
In addition. I wouldn't be surprised if certain brands insisted on higher quality quality of panels versus others because i.e AliExpress has sellers claiming to sell same panels known to be used in certain high end monitors but sometimes at a discount and the cynic in me wouldn't be surprised if they ended up being "B grade" or thereabouts from same factories.
On a related note NEC and Panasonic largely divested themselves of the high end monitor market a few years ago, so that's another potential curveball towards quality.