I can’t say for sure whether or not this particular study used proper testing, but as a whole introversion and extroversion is not pseudoscientific.

Jung wasn’t a good scientist, but he did a lot of studies and came up with a lot of theories, some of which happened to be at least partially correct. Also, you seem to be getting something mixed up because Jung defined introversion as an “attitude-type characterised by orientation in life through subjective psychic contents”, and extraversion as “an attitude-type characterised by concentration of interest on the external object”, whereas the more common energy focused definition is not from Jung at all - at least, as far as I am aware.

The big five personality traits, namely openness, conscientiousness, extraversion, agreeableness, and neuroticism have been shown to be consistent, even cross culturally.

There are limitations to that: like how it’s an empirical observation, that other personality traits exist that aren’t factored into those five, or that it’s possible there are a larger number of smaller subfactors that make up those five traits, but ultimately they are scientific.

Except by that exact source, mass shootings with rifles are under reported and the deadliest mass shootings were done with semi automatic rifles.

“Since 1982, there has been a known total 65 mass shootings involving rifles, mostly semi-automatics. This figure is underreported though, as it excludes the multiple semi-automatic (and fully automatic) rifles used in the 2017 Las Vegas Strip massacre – the worst mass shooting in U.S. history, killing 58 and wounding 546. In fact, semi-automatic rifles were featured in four of the five deadliest mass shootings, being used in the Orlando nightclub massacre, Sandy Hook Elementary massacre and Texas First Baptist Church massacre.”