I'm not a physicist or anything, but the idea I'm getting from that is that the event horizon of such a micro-black hole would be much smaller than a subatomic particle. It's too small to actually draw anything into it from a distance, so it would only be able to absorb something by running into it. The rate of consumption can't increase until the mass gets large enough to actually draw matter in from a greater distance than something like the size of an atomic nucleus.

The article author calculated how big that would be, and the black hole would reportedly need to accumulate about a billion tons of mass before it could start to grow exponentially.