Q is literally 'source' - with unknown author and unknown content - so we don't know what's in it nor what's not in it.
Therefore I challenged the claim by parent that 'anything not in Q ...' because that's an objectively ludicrous statement.
My 'theory', inasmuch as I have one, is that the authors of Luke and Matthew had different target audiences, different incentives, but predominantly the same (probably two sets of) materials.
They made different mistakes than the author of Mark, and indeed each other, though were presumably much more earnest.
The author of Mark was probably just writing allegorical fiction.
The author of Matthew for instance was the only one to talk about saints / zombies rising from the graveyards and walking into town, being seen by many people, etc - which is an odd thing for everyone else to have missed or not think to be worth mentioning.
There's a beautiful graphic showing % breakdowns of the Mark / Luke / Matthew content about half-way down this page: https://rationalwiki.org/wiki/Gospels
EDIT: that whole page is worth a read to get a better idea on the bleed between the different texts, a historical context for when they were probably written, and a reminder of the earliest copies we can reliably put dates to.
I think earnestly citing RationalWiki is enough to give me pause to take the rest of your 'theory' seriously. You know it's a humor website for teenagers, right? It's essentially ED for a different kind of edgy post-reddit nerd, it's not really a serious place to learn about theology.
I was defending my earlier logic (we don't know what's in Q, therefore we don't know what's not in it). Have you identified a flaw there?
I would prefer to earnestly cite Price, Carrier, Fitzgerald etc, but those citations are less convenient, especially for casual readers (clicking a link rather than obtaining books).
Disliking the style of rationalwiki seems insufficient reason to discount the clear assertions made, explained, and with original sources cited on that page.
Can you identify matters of factual error in that graphic I pointed at, or other material on that link?
I note this is the only comment you've made on the entire thread. I don't believe it moves us forward.
The simplest inference is that Luke simply cribbed what he liked directly from Matthew, and omitted what he didn't.
There is not a hint of evidence for unattested "sources" beyond their fevered imaginations. Making shit up was just how the stuff was produced, at the time, as illustrated by the (literally!) dozens of other "gospels" and forged epistles from the same period.
