The printed English rule book says "If the director of a company possesses a tunnel construction right, then in addition the company can (according to the normal building rules) build a tunnel on a marked tunnel hex." I think the bolded portion incorporates the requirement that a "tile laid must be an extension of track connected to a station market of the company." (It may also be worth recalling that in the original version, a tunnel was represented on a transparent tile, rather than a piece of cardboard.)
That makes sense. Thanks for pointing out the relevant text.
This leaves unanswered the question about the FAQ on Helmut Ohley's web site. The answer given there would be an exception to the normal building rules. Perhaps someone at Lookout could answer whether this exception is intended to apply to the new edition?