Somewhat-Reticent The article pointed out that, contrary to a poultry farm, quarantining is not feasible. If it were possible to build an additional barrier around the land, such that they cannot leave and wildlife cannot enter, pending additional testing, we may be able to learn something from this situation.
On the other hand, the CFIA may well know better. It could be that this is exceptionally virulent, and even that would be unsafe. Otherwise, I would suspect that additional testing would be cheaper than compensating the farm owners following the cull, if there’s a reasonable likelihood (or possibility) that the illness has passed.
The problem is that, as @Thane_Bitter has suggested, we cannot know for sure that the owners can be trusted. The article points out that outside visitors had been allowed in. I haven’t read that, but @Thane_Bitter wrote that some of the birds may have entered the food system or been used for cosmetics, which could be bad news. If they can’t be trusted to act responsibly, then maybe a cull is the necessary last resort.
On the subject of infected meat entering the food system or being used for cosmetics, as long as handling precautions are in place, my understanding was that high-temperature processing is enough to kill the virus. My completely unqualified stance is that that might not be a problem, as in cooking or high-temperature rendering to make pet food. However, reports state that this common strain of avian flu is almost universally deadly in cats, and causes illness in dogs often requiring an expensive veterinary stay, so raw-feeding would be dangerous, and the possibility of mutation may be the reason that they are not willing to consider anything other than a complete cull.
It seems a shame to kill the total population of 400 birds; the question is whether it would really be safe to do anything less. Then again, if we can learn something now, before it makes its way to humans, might it be worth the risk, as long as that isn’t encouraging the mutation that might lead to the jump to humans? This is one instance where I am glad that isn’t my decision, and I have to blindly trust those who are entrusted to make them.