Maybe it was true at the time, and then they thought it through and decided for civilian stacking
While that is as possibility, I'd wager instead that the author of this article got the message mixed up. Given the concept of stacking units while transporting across water doesn't make much sense for the reason I've already stated, the mix-up seems to be a higher probability.
It would be easy to hear that civilian units can stack on military units (as well could transport units on sea military units) and think that meant that civilian and transport units can stack on their own kind.