Sorry, are you saying there is relatively little thermal resistance and no electrical insulation between heat source and tip?
That is somewhat the case, except for electrical insulation. But it is a bit more sophisticated: the tip basically _is_ the heat source.
I don't know the PS series, they use a much lower frequency (455 kHz, iirc), but only the MX series at 13.56MHz. But the principle is basically the same, i think.
What you see as soldering tip extends a tiny bit inwards as a copper slug. Around this slug is a cladding of a special alloy. Into that alloy you can induce a magnetic field, but only if it is below the Curie-Point temperature. Once it reaches that temperature, no more power can be induced into it. Around that is a coil (insulated wire). The RF is applied to that coil.
That construction basically is a transformer. You feed RF into the primary, while the secondary is just a single-turn short. Feeding in energy heats up that secondary, of course. Since that cladding is in direct contact with the copper slug/tip, the heat is transferred (basically) instantly into the tip. Curie point reach -> no more heat produced. Dropping below Curie point -> induction starts again.
So the temperature control here is based on the physical property of that special alloy. Since all that is tightly thermally coupled, and very small in physical dimensions, heat recovery is almost instant. The real trick in these units is to control the power that the RF final is generating. RF has the nasty habit to be reflected back into the output amplifier if there is a mismatch between source and load impedance. Naturally, as long as the tip is cool (and thus RF energy is consumed), little is reflected back. But once the temp is reached, there will be a huge mismatch. In normal circumstances it would destroy the RF output stage. Basically the same as transmitting with a HAM/CB radio but no antenna/dummy connected. The Metcal (and Thermaltake) units detect the amount of reflected RF energy and regulate the supply voltage of the RF amp accordingly.
So, what they do is to include the amount of reflected RF into the regulators feedback-loop.
[shameless plug]
In case you are wondering, i have just recently designed my very own RF supply for the 13.56 MHz system from Metcal:
https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply. But please note that this is _not_ compatible with the PS series.
[/shameless plug]
Greetings,
Chris