I suppose the key words are "without massive tactical advantage".
Well, there is also as far as I know no experience whatsoever to suggest that it is normal to suffer much greater losses when on the offensive unless you possess a massive tactical advantage. Since we were speaking of "assaulting a well-fortified enemy", take for example the Verdun and Somme offensives during World War I. Both were failed set-piece offensives against a heavily fortified enemy in an age where offensive tactics were famously disadvantaged and inadequate, and in both cases the losses of the defenders and the attackers were in fact nevertheless roughly similar. The losses of the Western allies when they were advancing in Africa, Italy or NW Europe during WWII were never anywhere close to the relative levels experienced in the East. And, the Red Army did not achieve a better level of relative losses when it was defending than it did when it was attacking. In fact, I cannot think of even a single historical example that would suggest that incurring 3-5 times the losses of the defender is in any way something that can be reasonably expected as a result of having to attack a prepared enemy. On the contrary, everything points to the opposite conclusion.