The article reads as a bit naive, but there's nothing necessarily wrong with linking to competitors.
In a lot of niche and cottage industries, competitors are happy to set up link exchanges. As these are from established and very on topic sites, it tends to be one of the rare instances of where I'd actually recommend considering a link exchange seriously.
While a number of these small industries will have some stock overlap, there is nothing wrong with recommending another website for anything not being stocked - in fact, the more niche your industry, the more likely a page showing links to different stockists can actually became a useful informational page for book marking in its own right as opposed from being just a link exchange page.
Also, independent shop keepers on the high street will often recommend alternative stockists if they cannot help.
2c .