Directly from this and in direct contradiction with Diana Walstad's claim in her book:
"While one might expect NH4+ to be preferred by plants, as its assimilation requires less energy than that of NO3-, only a few species perform well when NH4+ is the only, or predominant, source of N. By contrast, most species develop toxicity symptoms when grown on moderate to high levels of NH4+ (1-6), whereas normal growth in these species is seen on NO3-."
When applying a routine to supply nutrients, it's likely best to suggest that mostly NO3 and a little NH4 from fish waste, perhaps decaying plant parts etc is the best solution.
There seems to be a temptation to assume there is "a preference" and it can be applied to all plants. There is no support for that speculation. the other issue left out oif the disccusion that you bring up is how much are we talking about?
0.1ppm? 0.5ppm? 1, 2, 5ppm?
What's safe for the other critters like fish and shrimp?
What about bottom fish that right next to the soils?
Further, if you consider the NH4 issue as far as rate of uptake specifically in aquatic plants, at less than 0.5ppm of NH4, NO3 uptake rate is faster in Elodea. If you use rate of uptake to suggest plant preference, then the answer is no, NO3 is preferred at less than 0.5ppm of NH4.
But preference is not merely defined to a few hours of uptake studies, you would also need to consider growth totals with various NH4 and NO3 ratios.
Grow things out for several weeks and take the relative growth rates from the dry weights.
So unless you plan on having rather high levels of very toxic NH4+(to fish let alone plants), this notion has some serious flaws in support.
The references for support should match the conclusion
That was not done.
So in answer to your question, yes, for many species, high levels will kill the plant, but other factors such as temp and pH play a role, as well as other ions present in the water to balance the internal ion charge and ability of the plants to balance those charges.
So it depends a lot on many things how much is required to kill plants, but high levels will certainly "harm" them(less growth, more respiration etc). Rice and some other plants can handle, or tolerate high levels, but it does not "help" them.