Also, given the significant brightness variation across characters

I don't think that that's a compression or scaling artifact

I'm guessing that someone used some kind of pen with continuous ink flow. Not a ballpoint or rollerball

those dispense ink at a fixed rate as the point moves across the paper, but a fountain pen, marker, or brush pen.

I've never used one, so I don't know how much brightness variation one can expect, but I'm wondering if it might have been done with a brush pen. The stroke width is near constant regardless of stroke angle, so I don't think that it's a fountain pen, but then there are some finer characters, like the commas and backticks, and it looks like brush pens are capable of doing that.

It sounds like Japanese users use brush pens to do Japanese characters.

I think that at least some of those characters are katakana, the "alphabetic" form of Japanese writing:

Which suggests that the artist who crafted the text was also Japanese. They might also have been the one to do the brushwork/penwork, if it was a brush pen (or maybe two Japanese people, one designing the original text, the other doing the brushwork).