If "highlit" is not the past participle of "to highlight" then what is? And why?

Your use of "highlit" assumes that "highlight", because it contains the root word "light", can and should be treated as though it is the same as the word "light" standing alone.

But the "light" in the word "highlight" is used metaphorically, and the 'lighting' is done, not with electricity or combustion, but with a "highlighter" which deposits a yellow smear, or smear of another color, which 'highlights' selected text.

Thus, "highlight" is a new word which is not bound by the variations appropriate to the word "light" standing alone.

"Highlighted" is used in preference to "highlit", not only because it sounds more fluid [please excuse the pun], but also because "lit" places undue emphasis on the literal meaning of the root word "light".

Of course, there is no absolute right or wrong to this, Father Steve. The general prejudice in favor of "highlighted" is intuitive, not canonical.