Always worth mentioning that I'm not the only physicist who hangs out here and for the record astrophysics isn't my field, still I'll give it a go.

First, take a look at this image and this one this one

The next thing to recognize is that the ISM 'interstellar medium' is a very loose term. Our sun is located in a region known as the Local Bubble, a place that is a few hundred light across and very low density as gas goes, having been cleared out by supernovae some 20 million or so years ago, which is roughly how long we've been in it. A more local phenomena is the Local Fluff, a gas cloud a few dozen light year across moving roughly out form the galactic core which we have been pass through for a few score thousand years and will be leaving in around a score thousand years. We're not in the center of either, and in fact our near neighbor, alpha centuari (which is close to the galactic core) isn't in the Local Fluff at all, though many other stars are. The Local Fluff isn't that dense either, but it is several times denser the the Local Bubble average.

That's important because the various helio pause/sphere/sheath/etc are defined in terms of our sun vs the ISM, we plow through the stuff and it's density, relative velocity, etc determine the geometry and radius. I want to emphasize that because using it as a 'solar boundary' is especially arbitrary since the thing will change a lot when we leave the Local Fluff in 10-20,000 years and everything that orbits the Sun out at that distance has orbital periods of around that long, it's a very temporary boundary in terms of the things near it.

All boundary lines in astronomical terms are pretty arbitrary though, but the upper boundary would be our Hill Sphere, the ares where the Sun is the dominant gravitational force, and that's comfortably over a light year, whereas Voyager 1 is barely over a light day out. We do have long period comets that are closer to a light year than a light day, they're not uncommon, and the hypothetical Oort Cloud ends somewhere in the 50-100,000 AU range whereas Voyager is around 100 AU out. There are plenty of documented comets that sneer at 100 AU when they pop in every few hundred thousand years or so and which will encounter the heliopause at a different spot each time they pop in, which they've done hundreds or thousands of times. It would be all sorts of cool to get ice core samples off those. And those are just the comets, the guys with the highly eccentric orbits which probably are a small minority of the crap floating around our sun including galactic hitchhikers and other oddballs.

In any event, it isn't a disagreement about what the solar system's boundary is, everyone in the know recognizes that it's a pretty arbitrary concept and it isn't disputed like whether or not something is a mammal or not, it's more like fruit and vegetable, is a tomato a fruit or a vegetable? And the answer is that fruit/vegetable is just a bad old pair of terms.

This happens a lot, for instance there are no 'Gas giants' in this solar system, they're Fluid Giants, brown dwarfs are really purple, and pluto wasn't demoted from planet status, we just call it a dwarf planet, much as we call Jupiter a gas Giant planet, and we also call our own star a 'Dwarf star', we certainly didn't demote Sol from star status. The Heliopause is basically the boundary of the Sun's magnetic and thermodynamic dominance, but orbits are based on gravity and thus gravity, and the Hill Sphere of the Sun, are better definitions really.