According to an Apple support document, the company is claiming otherwise. Apple says that the new iPad Pro is compatible with the first-gen Magic Keyboard, but it might not offer a precise fit. “The first generation of the Magic Keyboard (A1998) is functionally compatible with the new iPad Pro 12.9-inch (5th generation) with Liquid Retina XDR display. Due to the slightly thicker dimensions of this new iPad Pro, it’s possible that the Magic Keyboard may not precisely fit when closed, especially when screen protectors are applied.”
This means that if you do have the first-gen Magic Keyboard and want to use it with the new iPad Pro, you can, just that if you have a screen protector in place, it might not close as well as you would like. If you typically use your iPad at home, we imagine that this might not really be that big of an issue.
However, if you do take it out quite often then the not-so-precise fit could potentially lead to issues like it slipping out, having certain parts of the body scratched and scuffed, and so on. You could always try fitting it before deciding if maybe a new keyboard case might be warranted.