Ex66,
Interesting question about duel worthy, OV. I don't know is the short answer. I don't really duel with my sabers, that's mainly why.
I'm not convinced about dueling with the Day Blades. The photo I just took shows it is about 40% of the thickness of a Saberforge Infinity V4 blade (judging by eye only). You might be right if you use identical blade types to duel with it, Day Blade with Day Blade. But against the SF Infinity blade, Day Blade probably has no chance. Also if you squeeze it with two fingers moderately hard (index and thumb say), you can actually see it move under the pressure.
My error mate, when you wrote:
It's worth nothing that the blades in the video are not designed for dueling though. These two blades are very thin-walled.
I actually concluded that the disclaimer stemmed from user experience. I haven't actually seen any videos on YT demonstrating it (yet) however when both blades are from same make and origin I would think that they are duel worthy. There's several video demonstrations from each Blade Vendor showing their respective superiority and durability (and also compared to the competition) but these are typically set using their Heavy Grade Duel specific variations (which as a generalisation) may contribute to the inconsistent diffusing of light (in some cases).
Saberz with their Starlight Blade claims to be a thin wall and boasts consistent illumination (characteristic of thin walled blades) whilst remaining durable in heavy duelling (characteristic of Heavy Grade blades).
On VV's user comments for their Day Blades, one Reviewer commented on suffering just a few scuffs during aggressive duelling. Granted they're words on screen and remain unsubstantiated but I'm sure VV would have commented otherwise if their Day Blades weren't suitable under the alleged conditions.
My experience with Mid Grade Blades (other than a slightly brighter illumination) have returned that whilst accurate to your comment about 'finger squeeze' they still hold their own even against a Heavy Grade. The advantage whilst being marginally lighter (that contributes to improved balance rather being top heavy) is it promotes a virtual whipping motion with slight flex.
Imagine a piece of bamboo.
Am waiting for some Day Blades to come in (specific to VV) to determine if they are equally as durable and able to withstand against contact with an Infinity V4. Noted there are other Day Blades available from other Vendors also but I don't plan on doing an official product test with all that's available on the market.