The way i understand it, Dorion will only be allowing teams to talk with DeBrincat's camp about an extension once the basis of a trade is agreed upon. So basically Dorion would be negotiating with interested teams on what a trade would be for a signed DeBrincat (without a new deal actually being in place yet), and if he's able to agree to the basis of a trade in principle with a team, that team would then be able to talk with DeBrincat to see if they can agree on an extension. If the interested team later can't agree to terms with Cat, then the trade that was agreed to falls through and simply never materializes.
Allowing the process to play out in this specific order is the right play in Dorion's position IMO. Permitting teams to only speak with DeBrincat after agreeing on trade terms protects the Senators' leverage in their trade talks; it prevents other teams from altering their offers if they feel extension negotiations with Cat are in their favor, and it also prevents Cat from dictating the strength of the offers coming from the interested teams.
Just as an example, let's say that Detroit, Dallas and Carolina are all very interested teams and they can speak with Cat and his agent before talking trade with Ottawa. Cat and his agent tell the teams his price is $8.5M x 8 to Detroit, $9.25M x 8 to Dallas and $10.5M x 8 to Carolina. In this situation, Cat could heavily influence or steer trade negotiations towards his preferred destination(s) which then impacts the return the Senators can get in a trade. The overpriced contract to Carolina can cause them to no longer be interested in a trade, decreasing the number of suitors and/or chances of a bidding war. Detroit catches wind through discussions with the agent that they have the most favorable AAV on an extension so they are no longer inclined to make their strongest trade offer to Ottawa. To maintain leverage in any potential deal, Dorion has to negotiate what a trade would be for an extended DeBrincat first, with the trade being contingent on an extension getting signed, and only allow teams to speak with Cat's camp afterwards.
I know it's not NHL, but I remember reading that the Houston Texans proceeded the same way a few years ago when they were shopping Deshaun Watson. Watson had a bunch of off-field issues going on and a no-trade clause and the Texans still got a great return when they eventually traded Watson. I don't have any NHL examples that come to mind right away, but I would imagine that there is a precedent in NHL trades too where this approach worked successfully. Dorion doesn't have the best track record, but he's handling this part of the process correctly.