Yes- "in transit" means passing through - i.e. - you are not leaving any of those items in Canada. I took a couple rifles, with ammo, Outside a couple years ago. No problem - just follow the guidelines/paperwork.
Ditto on VEK's comment on Border officials' attitudes, in general. Never had a surly Canadian, which is not true for the American side of things, by about 50%.
As to "why not buy them here?" - if he has them already, why leave them behind? Maybe cheaper also. Dunno about that. But getting Hazmat (primers, powder) up here by commercial carriers in bulk via "good deals" is a PITA, and very, very expensive, especially in smaller quantities.
Getting such items out into the bush requires a minimum of $50 hazmat fee per shipment, on top of the air transport fees. Likely the same from Outside. And it cannot fly oncommercial passenger aircraft such as Alaska Airlines. Only on cargo-only flights. The puddle-jumper commuter aircraft within the state may be different. I don't know about that.
Again, I am ignorant about fees for barge/truck "ground" shipments, not having used such for a couple decades. I don't know how the wholesale/retail outfits up here get their Hazmat, other than it must come "ground" - probably via truck to Seattle, then barge, then truck again to destinations. Off road system it would again have to go cargo only aircraft, or barge on the river systems, say to Bethel or Nome.
One thing- "Pepper Spray" is not allowed into Canada. When asked about it at the Alaska Border, heading down, my wife admitted to having a canister in her fanny pack. Canucks filled out the confiscation paperwork while she went out to get it. On exam, they said- "This isn't Pepper Spray- it's Bear Spray - right there on the label", and handed it back to her. Then they had to do more paperwork justifying the cancelation of their original paperwork.....
. Nice folks.
