"I was taught that the EM spectrum frequency range is essentially infinite in both directions. So 35% of the particular infinity below 15GHz doesn't really make things very clear."

When people talk about 15GHz they mean either +15GHz or -15GHz. There is no measurable difference between them until you start talking about relative phase, and I'm guessing you didn't get to that chapter yet.

So 35% of the spectrum below 15GHz means 35% of the spectrum from 0Hz to 15GHz or 35% of the spectrum between -15GHz and +15GHz. It does not mean 35% of the spectrum from 15GHz down to minus infinity.