I think the right way of reading it is to put consider the outsider the founder and otherwise consider their takeover as the founding. It would be like Bill Gates buying a 4 person company that made Basic interpreters instead of starting one, it's not fundamental to the story.
If you looked at a graph of Starbucks and another startup going public at the same time the would look like this
. . . . . . . . . . . . . . . . . . . . IPO
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. . . . . . . . . . . . . . . . . ../
. . . . . . . . . . . . . . .founding
. . . . . . . . . . . . . . . . . . . . IPO
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. . . . . . . . . . . . . . . . . ../
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founding----------------acquisition/
What happened between founding and acquisition in company #2 isn't really part of the story. The day after aquisition though is pretty much the same after the day of founding company #1.