He might have said it under his breath.
Not to nitpick, but his lips were closed and he was breathing in through his nose - hey, maybe he was doing circular breathing!
Anyway, in order to give them the Bracha, it should be audible to the Chassan and Kallah.
You know those people that get an Aliyah and mutter the beracha under their breath? I guess they don't realize that they are really being Motzei everyone in the shul with their beracha!
Similarly, when a chazzan says Modim (and everyone else is saying Modim D'rabannan), they should either wait or say it loud enough that it's audible.