You're forgetting 2 out of 3 covenant ships were destroyed. Total covenant ships = 750. this means 500 were destroyed.
I'm betting those remaining 250 ships where crippled or damaged.
All the UNSC could muster was 152 ships, of which 130 destroyed. That still leaves very good odds.
The infinity would be Command and control. If the infinity was destroyed think about how many ships you could still fit behind her hulk.
If infinity was the last ship standing, Hit and run out of spaceship, deploy stridents to clean up.