No, they aren't. These red river hogs live in an enclosure most zoo river hogs would dream of. They have a huge forested hillside with plenty of bushes and other undergrowth, a lake and a lot of mud and sand to wallow in, so it is not that they can't avoid the ocassional lion at the fence if they were afraid of him (which they aren't). This young lion was watching them just doing their thing and they completely ignored him so I don't believe they are "living under constant stress".
In addition to being one of the largest red river hog exhibits I know (giving them plenty of possibility to move out of sight of the lions), the lion exhibit is also huge at 2,5 acres and it's only in a small part where they can peak into the red river hog exhibit. So, for much of the time the lions aren't anywhere near that section and in any case the red river hogs can always move out of sight. Both exhibits are among the best in the world for these species and reveal what can be done for relatively limited funds if you have a lot of space. Interestingly enough, this approach resembles the one at the Skandinavisk Dyrepark, which is only a few kilometers away.