What they are asking you to do is reserve the apartment for them without then paying immediately to secure it.
I would outright decline personally. If they wish to definitely secure the apartment then they must pay for it now. If they do not wish to make payment so far in advance, that too is their choice but they risk losing the apartment to someone else who is ready to commit. Their best bet is to check nearer the time they wish to travel/ are willing to make payments and take what is on offer at that time. They can't have it both ways. Also what's to say if you do agree to block off the date that they will actually book them nearer the time so you are closing your listing off to other potential guests with no security from these guests.
Inform them that this is how Airbnb works, they make a payment and the booking is confirmed. Yes Airbnb take the money immediately and do not release to you until 24 hours after the guest has checked in. Going by their message and reading in-between the lines, I also feel there is the possibility of them trying to get you to take cash payment to avoid paying Airbnb fees if they do proceed. Not only would that be a breach of Airbnb policy, you would not be protected by ABB in the event of any issues down the line; it’s not really worth the risk.
Personally, I don't like the sound of it, they are trying to be smart. I cannot tell you what to decide but if it were me, I would politely explain that there is no facility to block off dates for so long for a reservation; reservations can only be held for a maximum of 24hours, that they are welcome to check neared the time and if the apartment is still available, that you would be happy to consider their request to book via ABB at such a time.
BW, Ama