he was absolutely core in that the team would be making long term plans based on his projection as a top pair defenseman
If that's what you're defining as core, sure I guess he was core. He's really more "top prospect", but if you want me to really rephrase,
Teams don't trade young/prime aged controlled players putting up quality results at the top of their lineup (eg there is a difference between a guy getting top line minutes and sucking and one like edvinsson getting quality results) unless they ask for a trade or won't resign.
Teams also do not trade their recent top 5 picks unless they ask out, refuse to sign, or shit the bed.
Players like Buium, which if you want you can call core, are rarely traded, but can be seen as the centerpieces of trades on occassion without asking out.
In general when it comes to the draft, when you pick in the top 5 or so, you are drafting for your core until they prove otherwise
When you're drafting in the 9-15 range, you are drafting assets until they prove otherwise. Sure, they are very high end assets, almost always the best available assets you have and you have them as part of your future plans. But they get traded and used as trade bait.
It's why you see picks in that range traded every so often, and yet a top 5 pick hasn't been traded in the salary cap era.
Buium is/was at the very high end of pieces who actually get traded/shopped.
But he wasn't in the level of piece where they are simply untouchable in every negotiation, eg like Boldy or Faber