This type of finish was very popular 50 years ago. Unfortunately, as your expert has said, it is practically impossible to reverse and gain good results. The reason is that the wood is not just bleached, it is also coated with a light color of paint, usually cream, to make it even lighter and then finished. It's the paint on the raw wood which is so difficult, especially with oak because of the large open pours. The only way to darken it without stripping first, is to add a tinted transparent finish which would have to be professionally spray applied to ensure even color.