Why rsnyc's --inplace is not exactly inplace

I have been rendering my own tiles for a long while now. One of the problems of that is the amount of disk space this requires. Right now I not only have XXX GiB of tiles, I also have YYY GiB of DEM files on my home server. For this I have dedicated an old 750GiB disk I had laying around. As this is my hobby, I don't plan to expend much more money on it; I have already bough a 1TiB SSD that has already been replaced by a 2TiB NVME just to be able to render all over Europe. And I do have spare disk space, but to make it real I want to plan something else.

But this is not the point of this post. The point here is that right now that fisk is full. Not a single new byte can be written in it. But I tried to be clever and use rsync's --inplace option, and yet it doesn't seem to work. If I do my dialy render of my local area and then try to push the files to the server, I get¹:

rsync: [receiver] write failed on "/home/mdione/www/Elevation/10/524/391.png": No space left on device (28)
rsync error: error in file IO (code 11) at receiver.c(381) [receiver=3.4.1]