> At a high enough frequency, the signal will be indistinguishable from noise and 'stops wiggling'.

It's not that the signal will be indistinguishable from noise, but that the CPU will stop working correctly, so its outputs will stop toggling (or will toggle in unexpected ways).

I had an impression that they might be talking what a digital I/O pin looks on oscilloscope.

Scopes lock at first rising edge after last horizontal scan, so display starts at H, then drops to L after how long CPU held that pin high. That creates ￣￣l＿ lines on the screen superimposed to one another, X position “wiggling” ￣lll＿ depending on how many consecutive H bits just happened to be sent.

When the CPU halts, the pin would flatline at H or L and you’ll know.

I was actually thinking about catastrophic failure, as in something on the chip burning out and making the clock stop.

My guess is that they were probably cooling it with beer (or at least cold beer bottle bottoms) to get that last critical Mhz, before drinking the beer.