As you may be aware the TGV line from Paris to Strasbourg is brand new. As well as the halved journey times there is a substantially enhanced service compared to the old days. As a result it is unlikely to be in any way crowded.

Those fares are indeed very low for TGV service and doubtless are some introductory special deal.

If you are only used to air travel you will find that second class on the TGV has about as much space as first class on US domestic airlines. First class on the train is more roomy again, but there is no significant service difference.

The route may be new but the trains are not. Some new higher-capacity double-deck trains have been built for the existing and more heavily used TGV lines, and their trains built some years ago have in turn been handed down to TGV-Est.