Perhaps, by going in through other openings that some frames have (like holes for internal cable routing, the drain hole present in many bottom bracket shells, and of course the top of the seat tube).

But WD-40 is not a very effective corrosion preventive substance. It is excellent at displacing water, okay as a penetrant, fair as a lubricant, and perhaps better than nothing for long-term protection.

When I was assembling my bike a few years ago (a new steel frame), I recognized the easy opportunity to treat the frame. Since I bought a bare frame and built it up myself at home, there was really no excuse to skip this step. I did not buy frame saver, but just used linseed oil.

I don't know how much of a problem I've prevented--in other words, how much rust might be accumulating in an untreated frame. But I get peace of mind when I go out riding in the rain or especially on salty winter roads.

Linseed oil is not expensive, it's widely available, and it's easy to apply. As for its suitability, ever since airplanes were invented, it's what we use to protect their steel tubular structures internally. It's actually specified by the FAA for this use. So, seems good enough for a bicycle.