Yes, I noticed that and also that it is for Lazarus 1.9 which is not a stable version and won't come in near future as a stable version. So it isn't usable for me. Also I don't want to supply multiple icons for multiple resolutions. I just want to resize my current icons. Resizing TImageList images may not be needed just for High DPI scaling but for other purpose.
So back to my question. How to do that in multi-platform way? It seems that TImageList internals are pretty complicated. That it isn't just one combined bitmap but it also depends heavily on the widgetset. The main problem is that methods Add/Insert/InternalInsert don't work correctly with a transparent image under gtk2. And I don't know how to fix that.