in the wrong hands you can eat up the wood, create leaks, get mold from injected water and cause all sorts of problems.
I do see his point and I concur with wanting to clean the surface good and remove all unwanted stuff so that you get a good adhearson.
A 2500 psi power washer with a fan tip used about 16 inches from the work and working in a downward motion will not hurt the surface and will take away all debris and junk. BUT AIMING THE WAND UPWARD WILL CAUSE BIG PROBLEMS.
Your concern about oils and other have no bearing, he/she is wanting to prep the surfaces and this you have to appreciate.