Regardless of what Lipsius believed, he showed someone crucified on a pole also. This illustrates that stauros does not have to mean Christendom's cross.
King James Bible
And
Moses made a serpent of brass, and put it upon a pole, and it came to
pass, that if a serpent had bitten any man, when he beheld the serpent
of brass, he lived.-Numbers 21:9
compare
New International Version
Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up,-John-3:14