The article was pretty spot-on conceptually, but missed tons of details and supporting research. It looks like the author literally took a finance 101 textbook and summarized three chapters in one page.
In practice, liquidity doesn't matter on a day to day basis. But when it matters, it REALLY matters.
Just like your normal risk-premiums for inflation risk, return risk, volatility risk, etc etc, there is a liquidity risk premium. That is, how much added return do you get from holding illiquid assets? Depending on the situation, it can be significant. Or it can be nothing.
If you think of a residential real estate investment, you can earn outsized returns if you are able to buy from people that need to sell immediately. On the flip side, you get screwed when you're the one who has to sell immediately. If you are buying, holding, and eventually selling without any distressed sales involved, your returns from the liquidity risk premium will be negligible (assuming no changes to the underlying market premium).
Back when I worked in venture capital (many years ago), the liquidity risk premium on stocks was roughly 50%. That is, a public stock would trade at ~50% premium to private stocks. Now that Silicon Valley has gone bonkers again, some of the big name startups actually have a negative liquidity risk premium. Their private market price is higher than their public market price would be.