You either know or don't. If you don't, better don't fully rule out a possibility just because it seems unlikely or impossible to you (this is no license to start believing in stuff against your highest good, of course).
Plain wrong. That definition is more like that of the CID (current interrupt device) inside a cell, see Battery Safety 101: Anatomy - PTC vs PCB vs CID @ Battery Bro.
A cell protection circuit is a simple, built in battery management system. It consist of a battery protection integrated circuit (the DW01 is typical for single cell protection) which drives a few MOSFETs or dual MOSFETs (like 8205As and others) as a switch to control current flow to the battery. It servers as overcharge protection by limiting the absolute maximum charge voltage (overcharge protection), the allowable minimum cell voltage (overdischarge protection) and also protects the cell from excessive currents (short-circuit protection). A protection circuit, particularly in the case of small circuits like the ones built-in with cells, always take a toll at the cell's performance. Check, for example the following graph:
It is from HKJ's battery comparator, original tests performed on protected and unprotected NCR18650B cells in the old 2-terminal station. The difference between the red and the blue curves is the additional voltage drop caused by the dual 8205A MOSFETs in the protection circuit.
Particularly I don't use protected cells, I take great care of my batteries and flashlight drivers usually have all what is needed. In the case of simple unregulated MOSFET or linear drivers, the very minimum led Vf of white leds poses a hard limit to how deep the flashlight cell can be discharged, so no problem.