This may sound like splitting hairs but it's not so much all owners collectively but each individual owner affected (which amounts to the same thing).
What's happening here is a transfer of responsibility for common property from the Owners Corporation to individual owners. The mechanism for that is a kind of special resolution by-law.
Under a special resolution by-law, individual owners have to agree to take responsibility for common property. This is most often a trade-off for them getting something they want (such as being allowed to build a deck, as in the original email).
The point is, the Owners Corporation can't just decide, even by an overwhelming majority, to pass its responsibilities on to individual owners without their consent.
So this isn't really a unanimous vote, per se, but you would need to get all owners to agree to it, which amounts to the same thing. So your strata manager is right.
By the way, I'm curious to know what the problem is that would be resolved by your OC handing over responsibility for front doors to individual owners. I can't help thinking there might be a smarter, cheaper and more achievable answer than trying to get everyone in the complex to agree to a special resolution.