It will be short after it acclimates to your tank and doesn't melt or anything. You can then safely trim it whenever it get's too high. I think what they're talking about and this happens often with plants, when cut out in the open air it does something and reacts against the plant but when cut under water, it keeps it alive and healthy. Well alive is relative. But that's what florists do when cutting flower's, cut the stems at an angle and under water and it will live longer in a vase.
So I would assume that's more what's going to happen with the hairgrass. But once it's root and growing, you can trim it down wherever you want it to and it will grow back