Inkscape is a general purpose vector graphics program, designed to let you do anything.
Diagramming tools are there to provide you with a fast way to draw a diagram, which most importantly often contains connecting boxes with arrows that will auto-route and re-route when you move the boxes. I'm not aware of Inkscape supporting that, and I suspect that even if it somehow does, it'll require many more clicks.
Draw.io is good enough for a one-off usecase, but if you're creating a couple diagrams and consider your time to have monetary value, the better usability of Figma means it'll pay for itself really quickly.