It's entirely possible I'm wrong, but I've talked to several people about it and they've all read it the same way.
p.167 - The Infiltrate rule gives the unit the Outflank rule, if they're kept in reserve. And them being kept in reserves must be declared.
p.166 - 2nd paragraph of the IC description, "Joining and Leaving a Unit: An IC can begin the game already with a unit, either by being deployed in unit coherency with it or, if the unit is in Reserve, by you informing your opponent of which unit it has joined."
Rundown:
So far, we declare a unit with the Infiltrate rule is being kept in Reserves.
This gives them the Outflank rule.
And we declare an IC joining a unit in Reserve, legal as per the IC description.
Now that we've got a unit with Outflank and an attached IC in Reserves, we look to the Outflank rule.
p.168 - "During deployment, declare any unit with at least one model with this rule is attempting to outflank the enemy..."
p.166 again - 6th paragraph: "While an Independent Character is part of a unit, he counts as part of the unit for all rules purposes..."
Which leaves us with the following:
A unit containing at least one model with the Outflank rule, being kept in reserves, declared to be outflanking.
Sure they have the infiltrate rule, but they aren't using it. They AREN'T INFILTRATING. They're using the Outflank rule they've qualified for by being kept in reserve, and the IC is tagging along as part of a unit with the Outflank rule.
Make sense?