No actually, I believe them when they say they completely rebuilt them. I played FRL a lot (finished it twice, because historic Ferrari), and the versions of those three tracks in Pcars are significantly different.
Hockenheim, in Pcars is the 1982 version, with the first version of the Ostkurve chicane, although the third chicane is obviously too tight on entry, but then the motodrom is accurate enough to not have the run offs and the drag strip. In FRL, the old Hockenheim was actually labelled as a 1982 version, yet it had the last version of all chicanes, and the runoff and drag strip in the motodrom that was there right up until the track was butchered. This FRL version was identical to the historic Hockenheim in Pcars during development.
Rouen is vastly different in Pcars to the FRL and Shift version. Again, early development Pcars had a direct copy of the Shift and FRL version in it, which was significantly less accurate than the version we eventually got in Pcars, so good work SMS on that one!
Silverstone is, like the above, different to the FRL and early development versions, though the differences are more subtle. The trackside objects are much more detailed in the final Pcars version, there seems to be a bit more elevation around the bridge area, and the gigantic runoff at the end of the hangar straight in FRL is gone in Pcars.
If you watch videos from Pcars development, and videos of Test Drive: FRL, you'll see those three tracks are basically identical. However, if you then go and have a drive in Pcars on those three historic tracks, you'll see they are quite significantly different. This pleasantly surprised me when I drove them for the first time, and gives me hope for the vibrancy of the historic Spa and Monza tracks that are yet to come.