We have seen that corner-to-corner is very bad.
But one square apart is not much better! They need to be right up next to each other.
After the Red L5, the Blue/Red player is in a position where there is a 1-square difference between the Blue corner and the Red corner closest to each other. (Between the red and blue arrows in the diagram).
In other words, both Blue and Red share the same key square that they need to play in if they want to move forward, and so only one of them will get to do it (whichever one plays there first blocks the other).
This is a missed-chance at a connection (where Blue and Red touch each other so they can both advance in opposite directions). Red could have achieved the connection if the L was turned the other way around as in this picture: