No, that's not really true. A company goes to an OEM because it is more economical then making the product themselves. or they don't have the technology to do it themselves.
A company like Lexar will give Panasonic a specifications to which the product must conform. If it is a high end product, then the specs will be high and stringent. If the OEM is for a cheapo product, then the specs will be low. The pricing that OEM company gives will then depend on how high the specs are.
So it's nothing to do with OEM company giving the best product. It's just simply building to specifications. For example, those Lexar SDs that are OEMed from Sandisk are very slow, just like the original Sandisks. This is defintely not the best product in the market.