Most dictionaries seem to indicate that highlighted is the past tense for highlight, rather than highlit. However, we use lit as the past tense for light without reservation, with lighted appearing much less frequently. Why the difference? Why isn't the derivation of the past-tense verb consistent for these two related words?
To me, "highlighted" says that something has been accentuated so that it stands out from a background or the crowd rather than having been ignited so that it sheds light on everything else around it. There is a distinct difference in meaning. And since "highlighted" has been verbed from the noun "highlight" by present-day users, it is unlikely that they would conjugate it irregularly.
Contrast that with the adjective "backlit", which is used to describe something that has been lit up completely and evenly from the back (such as a poster, a display on a screen, or a keyboard) to show all the details even at night or in dark places. This follows the irregular conjugation pattern because it is related to the original meaning of lighting something up.
English has a mix of origins such as Germanic and Latin languages and when it comes to conjugating verbs you cannot rely on rules as easily as you can on purely Latin languages.
I would consider your example as an irregular conjugation. You are right - it makes no sense, but it's English and you just have to learn it - sorry. There are plenty of other examples in the language like this:
- breach and breached
- teach and taught