I not only focus on rolling resistance, there are other advantages for a folding bike and a not folding bike.

For a folding bike, 20" wheels allow to have wider tires that not only roll better but roll smoother on bad road surfaces and of road without having a too big overall wheel diameter that increases the folded size (I am pretty sure that Helix limited the tire width to 40mm to also have a not too big overall wheel diameter and keep the folded size small, the
Helix fork for instance has almost no clearance above the wheels).

Another effect of smaller wheels is reduced wheel inertia that gives a more reactive bike that accelerate faster which is interesting for city riding with a lot of speed variations due to crowded roads and lots of traffic lights.
Lower inertia is also good when there are many uphills and downhills, race road bike uses low rim height and low weight wheels for mountain races to reduce inertia and high height rims, heavier wheels for flat high speed races where cyclist are riding in grouped.

I have about four or five spare 507X40 tires, so not buying any more as I think I have a lifetime supply for my Airnimal.

But a quick search found that Airnimal lists a Kenda K-West 507X40 on one of their bikes, my folder is a much earlier version of this bike model that I built up with drop bars:
https://airnimal.co/products/joey/commute

Found that tire at a few suppliers, but would involve tariffs to import them.

And some years ago I bought some Intense tires as spares, have not installed them yet. But a five minute internet search tells me that the company that made Intense is long gone.

So, looks like 507X40 is extremely rare with only one tire model available, that is from Kenda.

The Kwest is also what Helix mount as standard tire but its a cheap basic rigid 60tpi city tire and its not listed on the Kenda website anymore.

Then I am glad I bought extras when I realized how rare that size is.

Thanks.

While searching for the Kwest on the Kenda website (didn't find it?), I found two narrow BMX tires in ETRTO507 the Konversion Pro 40x507 and the Happy Medium Pro CX 32x407 but both without puncture protection so not really for road use.

Thanks for checking, but I use that bike so little that I am sure I have a lifetime supply of tires. Not buying any more.