You answered your own question: CPUs are throttled down when there is too much heat. That happens all the time in a laptop. A desktop with enough air flow never has that problem (unless overclocked).
It depends on your definition of throttled. A CPU that was totally unthrottled would just sit at its boost clock under sustained load, but even desktop CPUs can't manage that, dropping to their base if too much heat is generated. Of course, there are laptops that can't even sustain the base clocks, but some have enough cooling to match that.
The base clock rate is typically defined as the sustainable speed. Intel literally calls Turbo Boost "algorithmic overclocking".
The problem is this base clock speed is given by the CPU manufacturer not the laptop maker. And Intel wouldn't know what kind of laptop it's getting crammed into. So careful definitions don't really help. Desktops are already big clunky things that have to be kept plugged in, I can trust them a lot more to deliver the CPU's promised performance. Whereas laptops are notoriously making compromises because customers tend to be very unrealistic about noise, battery life, etc.
