Two unscientific thoughts...
1. It obviously didn't kill the fish...
2. I've bitten the heads off of thousands of soft plastics over the years when they got too torn up to hold the hook shank. So far the only ill effect I've noticed is that I keep getting older.
Seriously, it does raise an interesting question but I think we're going to need a polymer chemist to answer it. Just the fact that they were still intact in the stomach of that lake trout suggests that the base material is more or less inert, but perhaps there's something harmful in the colorants.