Hi Susan. I technically suppose that this is a protective film for extending shelf-life of lettuce. It could be a waxy liquid and/or edible oil-based material sprayed on the leaves of vegetables and peel of fruits. It is kind a similar concept with the protection some foods (e.g. grains, cereals, biscuits...) from air/oxygen by strech films or plastic containers. On the other hand, some fresh-cut vegetables and fruits keep on respiration for a while and release specific gases such as carbondioxide, carbonmonoxide, ethylene etc. These gases are normally metabolic products but also accelerate the aging of vegetables and fruits. This is why color of banana becomes yellow / brownish and soften within a few days after you buy them as very green. You can get more information on the issue if you search about "edible films", "food shelf-life" etc.

I do wonder about the crispness of the leaf too even in luke warm water you would expect to see some wilting. So Ali Durmus your theory could be correct, but Lewis McCarter your thoughts have merit too.

I have some lettuce growing but its not Cos lettuce as shown in the video so I wouldn't be able to replicate the experiment.

I am thinking that agronomic conditions used in cultivating could induce such toughness to the vegetable. I have observed something like that in Telferia occidentalis which is a vegetable we consume here in South east Nigeria. The ones grown in a particular region/state are softer in texture than the ones grown in some other region/ a different state. In summary, agronomic conditions could have induced such things as was observed in the lettuce.