If you leave a gap in traffic other vehicles WILL fill it. You can see it every day as people will dive into gaps whether or not they actually want to be in that lane or not they still feel the urge to gain some advantage by filling that gap.
I’m not saying that leaving a gap is a bad thing, far from it, it demonstrates good road behaviour. The issue is that as a professional driver you should anticipate and allow for such behaviour, in this case the standards of both drivers has fallen below par, one driver it should be almost expected of, the other one not.