"Placing equal numbers on a diagonal does not reduce your options in any meaningful way, so it should be aimed for."

If it's on a diagonal from top left to bottom right there is no possibility to but another number around them, or is there?

Yeah, I didn't understand this either. If you've got:

3 . . .

x 3 . .

. . . .

Then the number you're placing at x has to be both larger than and smaller than 3, which is impossible. I've reworded the advice to only recommend the bottom-left-to-top-right diagonals, but I'm not sure how good advice that actually is.