>Flixbus definitely exists in Austria, but people generally take the train, which is much faster and more comfortable.
Not always true. The Graz-Vienna(Airport) trip is often quicker by flixbus than by OBB train.
Trains in Austria are quite slow , often travelling at the same speed as cars on the highway or often times even slower.
When the tracks allow it, the Railjet goes over 200 km/h. Vienna-Linz only takes 1 hour, which is about twice as fast as by car. Same for the new Koralm track.
Doesn't Flixbus cap their fleet to 100 km/h? I'd be surprised if that's higher than the average speed of most intercity trains.
Graz–Vienna is admittedly a bit of a special case, since the railway tunnel there isn't finished yet, so I could see cars/buses being faster. (The train makes up for that in views, though ;)