Seeing that oak trees are very common in Illinois forests, there is a little bit of truth to the statement. However, most people suggest morels grow under elm and ash trees too; which isn't surprising considering almost all forests in Illinois have many elm and ash trees.
Heck, I find morels in upland forests, ravine forests, well-drained bottomland forests, forests full of honeysuckle, cemeteries, near yard trees, and old pastures full of walnut and honeylocust.
To answer your question...I believe it is merely coincidence and I would argue with anyone who said otherwise :-)
