The series is named 'What If', not 'How Could'.

hypothetical
/hʌɪpəˈθɛtɪk(ə)l/
adjective: hypothetical
    based on or serving as a hypothesis.
    "let us take a hypothetical case"

    supposed but not necessarily real or true.
    "the hypothetical tenth planet"
    
    Similar:
    theoretical
    speculative
    conjectured
    imagined
    notional
    suppositional
    supposed
    assumed
    presumed
    putative
    made up
    unreal
    academic

    Opposite:
    real
    actual
    Logic denoting or containing a proposition of the logical form if p then q.

noun
noun: hypothetical; plural noun: hypotheticals

    a hypothetical proposition or statement.
    "officials refuse to discuss military policy except in coy hypotheticals"

Also, the idea of Randall Munroe of XKCD having to resort to clickbait is hilarious. He's gotta be in the top 10 most famous scientists alive.

To you, what is the difference between the moon being turned into a black hole and being replaced by a black hole with the same mass?

black holes can also be small, actually, it's based on a ratio of mass to diameter