I think Dekes' point is you can't compare their stats from the same year, because Eichel is a year older. There's (obviously) a lot of development that happens in a year, when you're comparing someone who's 16 vs 17.
So it's literally true that Eichel outscored Matthews 10-7 (in the same number of games) in the 2014 WJC-18 tournament. But Eichel was 17 years old, and a year away from being drafted. Matthews was 16 years old, and 2 years away from being drafted. If you compare their performance in the tournament at age 16 vs age 16 (in both cases - two years away from being drafted), and then age 17 vs age 17 (in both cases - one year away from being drafted), Matthews is ahead in both situations.
Maybe the counter-argument is you can't compare the different years (ie 2013 when Eichel was 16 vs 2014 when Matthews was 16). Maybe the tournament was much higher-scoring in 2014 - I don't know, I haven't checked. Or maybe the competition was tougher in 2014 - again, I'm not saying this is the case, just brainstorming ideas.
The point is - both posters are correct in saying that their numbers are accurate. That's because they're two different types of comparisons.