Are these simply PTH but with a zero-mm hole diameter?

Examples of JLCPCB capabilities show an actual TH.

erm... no. With a zero diameter hole how could they be internally plated to form vias? Vias are actually TH pads. Nothing special about them. I often use zero centre diameter pads, on one side of a board, when changing track width if there's a big difference as it acts as a stress reliever to prevent the track from lifting.

The vias we usually have are "tented". The solder resist form a "tent" over the pads. You can also have "filled" vias. There are two sorts, solder filled and epoxy filled. Both will cost you extra. The epoxy filled ones tend to be used on gear that has to withstand a lot of moisture and vibration, such as military gear.

Looking at Pete's RP2350DIL board. There are vias but with no daylight shining through.  

The vias are produced at the same time as everything else. He's probably used very small ones on that board. I tend to use 1mm dia with a 0.5mm hole, but they can definitely be smaller. When plated the hole diameter goes down by the thickness of the plating. Then the solder mask is applied on both sides. That tends to fill the tiny holes as well as tent over the pad. I've just checked and the 0.5mm holes in mine are clogged in solder resist too - I can't shine a torch through them.

  Mixtel90 said  

The vias are produced at the same time as everything else. He's probably used very small ones on that board. I tend to use 1mm dia with a 0.5mm hole, but they can definitely be smaller. When plated the hole diameter goes down by the thickness of the plating. Then the solder mask is applied on both sides. That tends to fill the tiny holes as well as tent over the pad. I've just checked and the 0.5mm holes in mine are clogged in solder resist too - I can't shine a torch through them.

Awesome. Thanks Mick   I was suspecting this because the holes do have a dimple.

Just pulled the gerbers into SL6 and found 0.3mm holes