It doesn't specify that all seats will be added to Avios redemption availability at T-355. That's purely an assumption which I agree would be fair to assume as this is the norm for when inventory is generally added. However there's nothing stopping BA from adding it in at a later date (whenever that may be) and still being within their own rules. So I wouldn't go all out on that limb just yet, it's entirely feasible they could be added in at a later date of BA's choosing..... when lower revenue fare buckets are added for example.