Function of "too" in the phrase "so too" or "so, too,"

Solution 1:

You phrased it well in your question:

the main point of too is simply to emphasize the also-ness of Event B.

In the immediate context of your example, the App Store is important because it strengthens the market appeal of the iPhone and the iPad. ‘Event B’ — hardware sales — is the star of the show.

The title of another relatively recent online article illustrates this well:

This land is your land... and so, too, is the ocean.

The ‘so, too’ here means something like ‘you already know that this is the case for land [from the Woody Guthrie song], but I want to talk specifically about the ocean.’

Solution 2:

So is part of the so X/such a(n) X that S construction. It's a quantifier, in the sense that it measures the degree of some predication. And it's a pro-form -- that is, it stands for something else, like a pronoun does for a noun -- and in the case of

... as the App Store’s fortunes rose, so too did the iPhone’s ...

it stands for what happened to the iPhone. The App Store's fortunes rose, and so did the fortunes of the iPhone. I.e, the constituent above "comes from"

• ... as the App Store’s fortunes rose, so too did the iPhone’s fortunes rise

by a sort of So-tag formation that

• pivots the clause around the so fulcrum,
• inverts the subject NP with a dummy Do-support auxiliary
• reduces the subject,
  by deleting (the repeated) material in iPhone's (fortunes rose).

Quite a complicated construction, with a lot of specialized syntax.
Note that one clause uses rose but the tag has to use did ... rise;
and the deleted material has to be totally identical to the material in the first clause.

This requires simultaneous parsing and pattern matching at oral speeds,
which is a bitch to program, and which is buggy in rapid speech and writing.