I would answer this by what Auston Matthews said in todays interview...
When I say best-on-best (or whatever similar term really) I mean just the amount of elite talent at one place. So, when you say "this tournament has more talent per team than the Olympics", yes, that's quite what I have in mind. The other thing is that the U24 players that are "missing" from Canada and US, are having their own team, it's not like they just cut the age for them, you know what I mean. I'm not saying I have one super-strict perfect definition. It's just what I feel about it. Are the best playing against the best, as Matthews himself said? Well, yes they are. I believe that players are pretty honest about this and don't really feel anything being so much "off" that wouldn't allow this tournament to be called best-on-best...I'm pretty sure that they are not so absolutely strict about the definition, and I definitely believe that many of them would agree with you actually, if you wanted to present the logical view on this, but they would likely added that it's still best-against-the-best, you know...
and when you say, everyone is using "this" definition...well it seems that players don't care much about the strictness of the word
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I don't mean to sound anything like offensive to you...that's just my thoughts...