Yeh it sounds like they just want to organise a temp so long. They're well within their right to do that.
It sounds cheeky but I'm sure it wasn't meant that way. I got lucky fortunately. My boss hasn't discussed any of this with me at all. When I went to see him about maternity leave the other day he stopped me mid sentence and said "No no. Don't set dates. You dont know how you're going to feel. Decide closer to the time. I know when you're due so it's fine. You can just tell me one day you've had enough and go."
So..... woohoo!!

LOL
Just give them an idea for now maybe? Tell them when you're due and tell them what your idea is now. At least that will help them organise their stuff.
Sorry just to add: I dunno how it works where you are. When I mentioned them being well within their right to organise someone to work in your place I was thinking about here.