Think about it...
That ship could have just left after the bomb was on the corvette. No other fighter patrols were coming in, because the one that came back was destroyed.
Did the frigate stay to mop up fighters?
Because I think that ship could have just left as soon as the team was on and all the covenant fighters were destroyed no?
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It was also covering the pelican. If the pelican would've taken fire, the bomb might've gone off prematurely.