Things to consider (as already stated), Iserbyt and Pidcock were dominant in Koppenbergcross, which may be the toughest race of the entire season, and they are both still young (and no powerhouses). That was only 2 days ago and it's not unlikely that this played a big part in their performances today. I expect both to be better next week, and maybe better than the opposition save Mathieu. If Iserbyt were as good today compared to Aerts, Sweeck, etc as he's been the rest of the season, he would have been able to follow, and who knows even beat Mathieu. Aerts has been subpar this season so far, Sweeck hasn't exactly been lighting up the peloton either, and yet today they were the closest rivals to Mathieu.
What am i saying, is that we could still see a few weeks/races where Mathieu doesn't necessarily dominate like last year. Mathieu clearly isn't in top form yet, because otherwise there is no way Sweeck could have finished this close. Iserbyt clearly wasn't as good as he's been this far, or he would have finished ahead of the pack (save Mathieu) by quite a margin. So, now the question that needs answered is: how long until Mathieu is in top shape, and how long can Iserbyt keep his form up (or has it already started to decay)?
If today was the start of Iserbyt's form regressing and if things are only going to get worse for him, then we're looking at another boring season with Mathieu dominating wherever he starts. If it was simply the result of going deep in the Koppenbergcross, but the form is still there, then we might still see some nice duels until Mathieu hits his top form.