It is worth realizing that the ARM core is not a primary part of the SoC, but an auxiliary core stuck on the side, behind a system MMU. The main (boot) processor is the VideoCore, which performs early initialization, sets up the system MMU and boots up the ARM core.
Most likely bcm2708 refers to the main part of the SoC containing the VideoCore processor and peripherals (look at wikipedia's table of VideoCore SoCs, notice how no bcm27xx part has an ARM core).
Based on broadcom's driver source code release, I actually get the impression all VC4 SoCs are based on the bcm2708, while VC3 is bcm2707. There are are at least three bcm2708 revisions (a0, b0, c0) though, and a0 differs sufficiently to have #ifdefs all over the place and quite a few headers of its own. There's also an exception for "VC4 big island" whose main header is not included, but I can't really find much about it, although this remark is an interesting one.
So it makes sense for drivers to be referring to the 2708 (especially if they were developed by broadcom).
I'm not sure what to make of the rpi2 reporting bcm2709... there are numerous statements that the bcm2835 and bcm2836 SoCs differ only in the ARM subsystem and the peripheral base address (i.e. slightly different system MMU config), so it seems highly likely it is in fact also bcm2708 based, but the info available is rather limited. I wonder if someone just bumped the number to deal with code that assumes bcm2708 implies an ARM11 core (while in fact it doesn't imply any ARM core at all).
Edit: The newly released bcm2836 quad-A7 doc confirms it is also based on the bcm2708.