GNSS RTK is incredibly accurate these days. By the time that you're close enough to the landing zone, you're close enough to get positioning down to centimeters on consumer grade hardware, which the article points out.

The actual question is literal: Can SpaceX land a rocket with sub 1 cm (1/2 cm) accuracy? GNSS RTK can get you down to a couple of centimeters, but getting more granular resolution than this isn't reliably possible with current professional grade technologies.

I'm personally unsure if the military has greater resolution than what's possible with RTK or w.r.t. military use GPS, but I would not be surprised if they did. If that's the case, NASA would most likely have access to it, I would assume. But the article specifically calls this out saying that it's not accurate enough to surpass the resolution of using RTK.

What's really cool about these questions is that the same problem space is applicable to self-driving cars and SLAM, if you're into that sort of thing. Lane detection, etc.