My 3.1mm screw had a stripped head, could not remove it. (Factory defect, this iPhone was never opened)
So I had to give up removing the display assembly, and jumped directly to battery replacement, Step 20.
That was tricky as I had to keep the display at 90º with a risk of breaking display connectors, but hopefully the replacement went well.
I would not recommend to do this but to follow the guide, but, in case someone encounters a similar issue, that could help!