Nature being to strong means Hume,in the terms you are using, thinks there are external objects.
We are affected by nature in certain ways. Things happen to us, all around us, within are experiences. The sceptic is destined to fall to what appears in our impressions. From beginning to end, someone can whine they don't have "proof" a billiard ball is about it hit them in the face, but then it does. Nature is too strong for our wish for "proof" to matter.
From here, we can actually extend scepticism to
a proof of external objects. The motivation of the sceptic is entirely driven by the existence of external objects. What is the monster we fear so much as a sceptic? Making a mistake, taking an unjustified/untrue position,
the world being other to what appears own our
experiences.
To participate in any sceptical project is a tact acceptance of the externality of objects. If there were no external objects there couldn't be a mismatch between what appears to us and what was true. Scepticism would be incoherent. It needs a world which can be other to our experience.
A world other than our experiences also has implications for how a scepticism can conceivably function. It must turn not only on certain assertions of what is true (as they might be wrong), but also on any sceptical rejection of a claim (as a claim might also be true). The coherent sceptic must, like Hume, oscillate between scepticism of one claim and accepting it (i.e. scepticism of rejecting it).
Hume isn't laying out a rejection of external objects, knowledge or philosophy. He's analysing the relationship of our reason to knowledge, trying to break with a philosophy which holds our reason or concepts are how our knowledge obtains (as in poor surface readings of Descartes and Berkeley).
We might say Hume is trying to recognise the life of the external world, that it is the things outside us which determine their existence, rather than us having an experience or concept. In this metaphysical space, he is constantly sceptical because he recognises our concepts are distinct from how something is true/made true.