That is absolutely FALSE. Strings very commonly play slurred notes with different bow strokes. So a slur in notated music does NOT mean strictly "take it in one bow."
Surprising explanation, William... A good composer or arranger knows about bowstrokes and wouldn't write something the players won't be able to play but this is another story.
A slur is a slur. 'Whenever a passage is slurred, all notes within that slur are performed on one bow, meaning that they all are played in one bow direction.' (Samuel Adler)
There's one case which is an exception: when a wide slur includes several bars i.e. It means the player should play as many notes as possible in one bow stroke and have soft bow changes in between. Furthermore, an usual way to get some continuous slurred effect is to split the global slurs between desks or players with different ones so as to spread the bow changes (and thereby rewriting the real slurs on the respective scores then).