Yes. There is a 2h15min high-speed train service between Paris and London via the channel tunnel (Eurostar), which is missing in the data. With connections, part of France is reachable from London, and vice versa. I guess the Deutsche Bahn data used to build this isochrone map is focused on continental Europe.

Graph theorists would call this a "critical bridge": if you remove this single Paris/London link, then a lot of indirect connections disappear without it. And the UK appears railway isolated from continental Europe, which it's not.

It is, but only from "Paris Nord", for which you have to zoom in and pick right next to "Paris Est". There are 4 or 5 train stations to pick from in Paris depending on your destination, and I'm guessing the added interchange time affects results (despite Paris Est and Paris Nord being basically 10 minutes walking distance for instance).