MRI showed bone marrow edema in my medial talus where I also have a visible bump. I'm confused because I thought the swelling is inside the bone?

Just like. other areas of the body, the bone marrow can respond to trauma, infection, or other insults with swelling/edema. This is usually only detectable on MRI. Because the outside part of the bone(cortex) is relatively rigid and doesn't expand when there is swelling in the inner part of the bone(marrow), marrow edema would not cause a visible or palpable lump.
Not a problem. The bone marrow edema on MRI simply reflects some changes in the marrow from your trauma, and would not be detectable except with MRI.