I have found out that we just needed to divide the third value by the width of the display, and the sixth by its height.

So, for our 1920x1080 display:

• weston: -1.234748 0.006603 2129.484619 0.003714 -1.155100 1153.294922
• libinput: -1.234748 0.006603 1.109107 0.003714 -1.155100 1.067866