How do they get good primary-secondary isolation on concentric windings?
There are two keys to making such a concentrically wound design work safely at mains voltage:

For most insulating materials used for the interwinding film, mylar included, DTI (distance through insulation) does not determine how much insulation is needed, and a single layer of mylar tape suffices as a result. Creepage (i.e. distance along the insulation surface) is the key factor here -- this means that the windings simply can't encroach closer to 4mm from the edge of the insulating layer.

Proper SMPS/mains transformer design calls for a special type of magnet wire, known as &quot;triple insulated&quot;, for the windings on one side of the transformer (usually the secondary side as that uses fewer turns to begin with), unless some other means of double insulation is used.  This wire has multiple layers of film-based insulation on the wire itself vs. the single layer enamel coating on ordinary magnet wire, making it more resistant to insulation damage and able to withstand higher surge voltages.  If it is not used, then the requisite double insulation is provided by other means, such as additional wraps of insulating tapes and spacers at the edges to ensure creepage limits aren't infringed upon.


As a result, properly engineered and constructed concentrically wound mains transformers are easily capable of meeting the same safety performance (hi-pot, etal) standards as their split-bobbin cousins, and provide the high coupling needed for an off-line SMPS to work anywhere near efficiently -- split-bobbin design isn't an option for high-frequency switching transformers.