By all means do use your Magnolia leaves somewhere as mulch or compost material, just place them correctly. Alleolopathy, or the inhibitory effects of plant chemicals on the growth of other plants is a real phenomenon in nature, but its effects in garden situations is quite over-stated. Most of the measurable effect, if any, is on seedling survival. Many people garden successfully in shade or part-sun under Magnolia trees, Eucalyptus trees, and even beneath dreaded Walnuts (Juglans spp.), which have become the poster children of allelopathic over-reaction.
It is true, the evergreen leaves of Bull Bay (Magnolia grandiflora) and other evergreen species will be slow to decay. Not the best for vegetable bed mulch, but ideal for mulching woody plant beds and garden paths, including the paths around the vegetable bed. The leaves of deciduous Magnolia will decay much quicker.