I think this might be where some of the naysayers are getting the opinion of fork extensions - from the old practice of using "slugs."
A properly machined and flanged tube extension should not give the flex that these pipes used to do with proper materials and tolerances.
Speaking from experience (I worked in a motorcycle shop in those days) the "slugs" referred to in the article were just like the "extenders" available today. They were fabbed from steel, typically chromed, and were machine-threaded on both ends; one end screwed into the top of the fork, just like the contemporary versions, and the other end was tapped to accommodate the fork-cap. They were available from several suppliers. We sold and installed dozens of them, in various lengths; they were a hot item. Everybody wanted their bikes to look like the ones in the movie "Easy Rider". I never saw any made from regular plumbing pipe but, doubtless, some back-yard, shade-tree guys did. I did see, on a couple of occasions, where guys would simply weld a couple of steel tubes to the tops of their forks; guess they were too cheap to have them properly threaded or to buy the commercial ones.
Now then, as I've related before in other threads on this subject, we stopped selling and installing them, following several bad accidents involving the front forks snapping-off, usually just after hitting a pothole or a rough railroad crossing. Since we'd installed a couple of the accident-involved units, we got really concerned about lawsuits. Fortunately, none transpired, but then, people weren't quite as sue-happy back then.
We DID start installing extended, one-piece fork-tubes, which became readily available not long after the slugs/extenders fell out of favor, and I never saw any of those break-off. However, some of the more extended units did put too much strain on the frame fork-heads, causing the welds to crack over time.
Anyway, the above is offered just FWIW..............
Cheers