As the phrase "dots per inch" implies, it's a ratio. Specifically, it's the ratio between your resolution (say, 4K) and the size of your screen.

What is "not true" is that every screen's pixel density is the same. Different screens have different sizes and resolutions, which means they have different pixel densities.

Who said that every screen’s pixel density is the same? What kind of bizarre claim would that be?

DPI is a resolution. It is not the ratio between resolution and size. It already has the units of resolution: 1/length.

Sorry, I badly misread your original comment: I read it as "I saw [a reference saying that] the two were the same," not "I've seen at least two displays with the same density."

Oh, OK. No problem.

You need to go reread the comment you called not true.