Where is the contradiction?
Its definitely not linear, you have areas to explore. There is a lot of backtracking, but you still progress through levels. Its closer to open world than it is linear. Because in linear there is only one direction and in bioshock you can take several. I didnt men to say it was 'open world' as such.
But the 'you couldnt get much linear' line is just not true because well, play most games, they have less freedom.
For instance you can go and get the incinerate plasmid first to melt the ice to finish the level, or you can go and get the telekinesis plasmid.