The quote is not really a good summary of his argument. The full Opinion elaborates much more on this, and his point is (from my reading) that it's a function of the "state" of the interaction between the police and the suspect, and what the definition of the word "seizure" means. I don't know if I find it convincing yet, but it's worth thinking about at least.

Full Opinion here[0], Dissent starts on page 22.

Full Oral Argument here[1], presented by the wonderful folks over at Oyez who really do wonderful work in cataloging the history of the Court (I'm not connected to them, just love what they do and I recommend folks listen to cases if only to sharpen your own rhetorical toolset).

[0] https://www.supremecourt.gov/opinions/20pdf/19-292_21p3.pdf

[1] https://www.oyez.org/cases/2020/19-292