Can you explain a little more how the comet speeds up during its trip?
Sure. What is true for comets, or any celestial object, is identical for falling objects we drop on Earth. The fall of an apple is how Newton began his famous gravity equation that became the first universal law -- a law that works throughout the universe.
So, since the Sun is so much more massive, objects will fall faster if they fall directly towards it. But their fall rate depends on how far away they are.
The strength of gravity determines how fast an object falls. This strength is strongest at the surface of a solid body like the Earth. But the farther you go, the weaker the gravitational field. It diminishes by the square of the distance. An object at 2x the Earth's radius, will feel only 1/4 the gravity as that at the surface.
I introduced two events because the news reports said Leonard had visited us 70k years before. Which I guess means that it has already reached perihelion back then and is now returning.
The farthest point in an elliptical solar orbit is aphelion; perihelion is the closest point. [It's easy to confuse these.]
The discoverer, Leonard, thinks the aphelion point from its prior elliptical orbit (implied) was about 3700 AU. Knowing this distance yields a free-fall time of about 40,000 years, which was the time he gave. [80,000 years for one complete orbit (i.e. period).]
Had he been able to tell how long the comet was traveling (i.e. 40,000 years), then he could calculate the distance at aphelion. Since it's reasonable to assume he had no way to determine how much time has expired for the comet, then his time estimate comes from plotting the known trajectory it now has backwards in time.
So how is it escape velocity now?
Right, a great question that deserves an answer.
If we assume that he has been able to accurately compute the aphelion point for this, now, hyperbolic orbit, then something would have had to give the comet an extra approx. 1 kps push or bump. This is easy to calculate since the escape velocity at 3700 AU is about 0.7 kps. If Leonard is on an escape route it has to be traveling faster than this once it reaches 3700 AU.
But what would be out there at that distance? This region is expected to be short of the inner Oort Cloud somewhat, but very little is known of the Oort.
It seems to come down to how accurate the aphelion point can be determined. If the 3700 AU value is close, then perhaps a rogue planet shot by and triggered a wave of objects flying all about.
But if the real aphelion point is 20,000 AU or so, then Scholz's star may have something to do with it.
Your assumption that something has to explain the extra speed that it never had before, again, is correct. Just falling close to the Sun will not allow it to travel to a greater aphelion point. If, however, you're a fan of "flubber", then there's that.