So it's the water coming out of the plant that (sometimes) reaches that level. The actual river has orders of magnitude more flow than that.
So he may have a valid point, but this is obvious FUD.
So in other words, the river itself might have a few tenths or hundredths of a percent of a concentration below the therapeutic MIC (potentially of multiple different antibiotics, depending on what factories happen to be located on that river).
Your interpretation of this is doesn't-matter, therefore FUD. My interpretation of this is enough to exert influence on relative competitiveness within a microbial community, and exert selection for antibiotic resistance.
Long before you reach lethal anti-microbial concentrations, you get subtle changes in growth rate and microbial gene expression. In agriculture, farms routinely use antibiotics at just a few percent of therapeutic dosing, and that is already enough to cause massive changes in the microbial community (with the side-effect of improving the growth rate of the host animal). You don't need to directly kill the microbes themselves, you just need enough to skew the balance of power between the various micro-organisms that are busy competing with each other.
The concentrations in the river may be a fraction below even that, but even slight pressures are enough to alter the course of evolution, when administered over a long enough time period. And "long enough" in this situation is in the context of an organism with 20-minute generation times.