None of that says or confirms that it's referring to his ability to use /outside/ forces, just that he's having trouble using any forces at all.
You say he's exhausted. That tends to mean he's lacking in /internal/ energy. He's worried that he's lost his magic, not that he's lost the ability to use outside magic.
That's not implied. Either one fits the statement equally well. Harry could have used up some inner 'reserve' of energy, yes, but he could just as easily be exhausted from manipulating outside energy. The overall trend of the series text is that most magical power, in humans, involves outside energies, except for death curses.
Yes, wizards /can/ draw in power from outside. But they also use power that's generated from themselves. What do you think Harry's doing when he throws together a spell from inside a circle?
Harry usually
can't throw a spell from inside a circle. When he does, the implication is that he's using what energy is there inside the circle.
Remember Aurora's circle. Harry was basically helpless inside it, the only thing he could throw was his death curse. Aurora did consider this last a potential problem, but otherwise Harry was more or less magically neutralized.
We saw this in action when Nicodemus trapped Harry, Ivy, Kincaid, etc. in a big circle, too. Because it was a big circle, there was quite a bit of energy in it, but Harry muses that once that energy is all used up, he's 'just a guy with a gun'. Unless, of course, he threw a death curse.