Sorry, I don't understand what you're trying to say. Why can't you guess the pattern by looking at the smudges?

Because the pattern can go back over part of the screen that's already smudged.

A valid pattern on Android is to swipe the middle 3 dots backwards and forwards 3 times like left-middle-right-middle-left-middle-right. The smudge on the screen just shows it uses the middle three dots some number of times.

Not on Android 9 (tested on my Nokia 7 Plus). Once you pass on a dot, you can't get back to it. You also can't "jump over" dots (top left - top right is actually top left - top - top right).

They changed that at some point. Back in the KitKat days, I once pranked a friend by changing their pattern to the same pattern as before, but by skipping the middle dot (top left -> bottom right). If you did it fast enough, the gesture looked identical to the old pattern.

You can pass over the same point more than once which would obfuscate the direction of the original passes on the smudge.

quick somebody do the combinatorics on this

The number of unique paths is theoretically infinite since the path could be infinitely long, but there's obviously some software limitation on that.