It's common to the point of mootness, but: when describing the planet, Earth is a proper noun and should be capitalized. Otherwise you are referring to soil.
The complaint isn’t about the colon in OP’s image, it’s the colon in OP’s explanation.
OP complaining about an insignificant capitalization mistake in a Twitter post, while making a far more egregious grammatical error in their explanation is just...*chef's kiss*
It isn't even an error, though. It's just wrong. "Earth" should be lowercase. I can understand being pedantic, but pedantic and wrong is super irritating.
The original post is technically correct on the topic of capitalization for "Earth". When referring to the planet, it is a proper noun, thus capitalized. source
However, that is a pretty egregious use of a colon XD
Keep in mind that idioms follow their own rules. Down to earth, what on earth, and move heaven and earth do not capitalize the planet, and four corners of the earth or salt of the earth take the definite article.
"On earth" (e.g. "richest man on earth") is one of those idioms, so the E is lowercase. See here.
on earth
used for emphasis. What on earth are you doing?; the stupidest man on earth.
It's a bit confusing, but they aren't literally referring to "the planet Earth" in an astronomical sense. If you were writing a sci-fi story and comparing the wealth of the richest man on Earth to, say, the richest man on Mars, then it would be capitalized.