It's in the article, but 25-35ms total. For the highest orbit they plan, one way latency is under 5ms, so these goals seem easily within the realm of possibility.
With multiple base stations that's the minimum latency to get across the street.
However, the added latency is not necessarily that bad vs landlines. Picture a triangle with the satellite at the apex and the goal at the other side. On the other hand if their base station was further from the network than your house then things get significantly worse.
With so many satellites in the sky, I think those effects will be exactly as you said, a few (<10 ms) milliseconds in the worst case, depending on how aggressive the hopping from sat to sat is.
However, the added latency is not necessarily that bad vs landlines. Picture a triangle with the satellite at the apex and the goal at the other side. On the other hand if their base station was further from the network than your house then things get significantly worse.