I think my prediction was 30-32mm for road as that’s where we begin to see the tapering off of rolling benefits on paved surfaces… but the show is unscripted, so might have said 28-30!
At this point, all of the safety data and everything we have on rim/tire interface, etc tells us that things get dangerous as the bead approaches the designed tire width. Remember, some of the air pressure in the tire is pushing the bead into place and also pushing the lower part of the casing radially inward toward the rim. Under ETRTO the bead width was extremely conservative… largely because beads stretched and tolerances weren’t as tight as they are now, but technically by ETRTO, the narrowest tire you can put on a 21mm bead width rim is a 35mm tire. In our testing, you can make a 25mm tire blow off of a 21mm bead width rim much more easily than it will come off of a 19mm rim and I haven’t tried putting a 25tire on a 25 rim, but I know that a 21mm tire won’t safely hold on a 21mm rim.
As a whole, the industry is still arguing the details here, my recommendation would be at minimum use a tire that is 4-5mm wider on the sidewall than the bead width of the rim for safety reasons, especially if using hookless beads.