Lemma 3.2.1 in Baker, González-Jiménez, González, Poonen, "Finiteness theorems for modular curves of genus at least 2", Amer. J. Math. 127 (2005), 1325–1387.
enter image description here
I don't understand why he says "all supersingular elliptic curves over $\bar{F_p}$ are isogenous". Could anyone help me?

Lemma 3.2.1 in Baker, González-Jiménez, González, Poonen, "Finiteness theorems for modular curves of genus at least 2", Amer. J. Math. 127 (2005), 1325–1387.
enter image description here
I don't understand why "all supersingular elliptic curves over $\bar{F_p}$ are isogenous". Could anyone help me?
This problem arises from Supersingular elliptic curves and their "functorial" structure over F_p^2.
Moreover, I found some similar question: Isogenies between elliptic curves and their endomorphism rings and Isogenies between supersingular elliptic curves.

Lemma 3.2.1 in Baker, González-Jiménez, González, Poonen, "Finiteness theorems for modular curves of genus at least 2", Amer. J. Math. 127 (2005), 1325–1387. enter image description here I don't understand why he says "all supersingular elliptic curves over $\bar{F_p}$ are isogenous". Could anyone help me?

Lemma 3.2.1 in Baker, González-Jiménez, González, Poonen, "Finiteness theorems for modular curves of genus at least 2", Amer. J. Math. 127 (2005), 1325–1387.
enter image description here 
I don't understand why he says "all supersingular elliptic curves over $\bar{F_p}$ are isogenous". Could anyone help me?