Summary: The article says that Bush is rejecting public matching funds in the primary so he can raise and spend more money, a planned $170M, compared to a spending limit of $46M for others who do take public funds. However, for the post-primary campaign Bush is said to plan to take public funds and limit himself to a $75M spending cap.
My impression is that in the general election phase, public funding is an all-or-nothing: if you accept it, the amount you receive is also your spending cap. (See FEC brochure here, though note that the absolute amounts have changed.)
Here's what I don't get: Bush has no competition in the Republican primaries. What's he going to do with that $170M, especially if he can't spend it in the general election campaign?