No, it won't change the leaf fall performance when that time comes, but it might mean that delays in the bedding in period are more closely aligned to the train's own issues rather than external factors.
If we are to take 'twelve months late' as suggesting that they now won't be in use until September 2022, two years after the direct award timetable, and introduced over the period between then and May 2023 then I guess yes, they could be introduced during the leaf fall period. Is that really what '800 driver' is suggesting. If so, it seems rather pessimistic from where they are at present.