No it wasn't...
μεσίτης
mesitēs
mes-ee'-tace
From G3319; a go between, that is, (simply) an internunciator, or (by implication) a reconciler (intercessor): - mediator.
Stephen wasn't mediating anything...
me·di·ate
verb: mediate; 3rd person present: mediates; past tense: mediated; past participle: mediated; gerund or present participle: mediating ˈmēdēˌāt/
1. intervene between people in a dispute in order to bring about an agreement or reconciliation.
You guys may be able to convince each other that your religion can change definitions to words but it doesn't fly with everyone...
Praying to God not to hold someone's sin against them *IS* intercessory prayer.
And that does not mean that Stephen (or anyone intereceding) is taking the place of Jesus, but rather, (as it were) acting as His agent, or in accord with His will. ("For we know that if we ask anything according to His Will, He hears us." / "It is the will of God that none should perish, but all come to repentance.")
Sheesh.