This, in my eyes, is what he is saying.

If a spirit is free, then they can participate on their own. However, if a spirit is keybound, then they cannot participate in the fight BY THEMSELVES. instead, they can ONLY fight when summoned by a celestial wizard.

in example.

Ursa, being a free spirit with no current keybindings, can freely join as a participant.
Gao, being keybound, cannot enter himself in as a participant.
Eli can summon his spirits into battle, which, in turn, allows Gao to fight.