It's a useful rule of thumb in terms of "where you'll stop accelerating with the wind on a normal boat", not absolute hard limit. You can play with length to start planing earlier which is the source of many a fun design (e.g. [1]). Some boats don't have a hull [2].

There's many other variables (weight, beam, etc.), here's a fun one: we once rented a 25 footer mostly because it was incredibly cheap (something like 150 EUR for a week). It was surprisingly slow for its size, even in fairly involved wind where we needed engine help to tack it had trouble breaking past 5-6 knots, to the amusement of not-much-larger yachts that overtook us like a German traveling salesman on the Autobahn. I took a dive under to see what was up with the hull and was faced with a scene not too different from the rocky bottom of the sea: barnacles, algae, some of them trailing foot long appendices...

The OP "yacht" is very strange to me as it looks like the guy put a keel and a sail on a buoy. The sailing dynamics appear to involve pitching forward instead of heeling (leaning sideways). I'm amazed he can do as much as 2.4 knots.

[1] https://www.youtube.com/watch?v=8CbJS_ZHs6g (Formula windsurfing)

[2] https://www.youtube.com/watch?v=Y7ie5C1-b3w (Mirabaud LX: foiler, without hull)