As with many marketing claims, this one has many holes.
First off, as mentioned before, if you verify part numbers at various retailers, you literally get the exact same part number whether you buy at Walmart or some other retailer. Whether that's a Goodyear Assurance or a Michelin Symmetry, etc.
Secondly, and also as noted above, larger retailers like Walmart, Sam's Club, Sears, etc have contracts with major tire manufacturers for retailer-specific models. The Goodyear Viva is sold only at Walmart (though its based on the Goodyear Regatta). The Goodyear Assurance Touring is sold only at Sam's Club, though it's based upon the Goodyear Assurance. The Michelin WeatherWise II is sold only at Sears, though it's based on the Michelin Harmony. If the claim is correct, and that's usually that mass market retailers get "seconds" or "blems", then where do the "firsts" go? If Michelin is only sending the "blem" WWIIs to Sears, then where are the "firsts" going? In the garbage? LOL
Lastly, sometimes these mass market versions actually OUTperform other general market tires. Case in point: the Michelin X Radial, like the WeatherWise II, is based on the Michelin Harmony. Back in 2005, Consumer Reports actually liked the X Radial the best of all the Michelin tires they tested; it performed better in many of the objective measurements. So once again, if these tires are of second quality somehow, it doesn't always show in measured testing.