I haven't read this page recently (it certainly has changed much), but on skimming through, none of the terminology McDonalds uses on their website are meaningful in the sense that the standards there is no official definition and could not be distinguished between similar competitors that make similar claims besides simply what they say, and, more importantly, there is no regulated term between what constitutes beef being “pure” or “not pure” or otherwise put. It’s just some “thing” they say about their beef and we have to take them up on their word. Now if they said “we use USDA organic ground beef” that might have some teeth. It’s the same thing between a bag of candy telling you they no longer use artificial flavors and now use “natural” flavors.
Now the question of do I believe they have changed? Possibly. Do the burgers taste different? No, so common sense tells me you don’t drastically change your process like this and still get the same tasting burger from 20 years ago.
But I will actually do something uncommon here and admit I could be wrong, and have an outdated understanding of their process.