I would have said "I can't see how this could be true, therefore it can't be true".
That's pretty much it, for the
Argument from Personal Incredulity anyway.
The
Argument from Incredulity "This can't be explained by X, so Y is true", in the broad sense, is identical to the
Argument from Ignorance (aka 'Negative Proof') "No evidence against Y, so Y is true", in that the
lack of contrary evidence is considered positive proof.
Because Jon is not saying that something is 'certainly true', then I can't see it being that fallacy specifically. He is merely saying which explanation is the most parsimonious, which is perfectly valid. It's very different from positing that "Independent evolution is too unlikely, so it must have been common descent", which
is fallacious logic.
See also: EvoWiki:
Argument from Incredulity (with a slew of creationist examples)
Edited by Doddy, : fixed formatting
"And, lo, a great beast did stand before me, having seven heads, and on each head were there seven mouths, and in each mouth were there seventy times seven teeth. For truly there were seven times seven times seven times seventy teeth, meaning there were. . . okay, carry the three, adding twenty. . . plus that extra tooth on the third mouth of the sixth head. . . Well, there were indeed a great many teeth" - The Revelation of St. Bryce the Long-Winded
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