I'm not from the US and was born after Greenspan took up the position but I thought Paul Volcker was a pretty well known name during his tenure?

During his tenure, yes. Volcker remains famous among finance nerds, because it turns out he was right. He caused a recession and risked his own job, but in doing so he successfully reigned in runaway inflation.

While Volcker was controversial in his time and celebrated later, Greenspan was widely respected in his day. I remember Greenspan being sought for interviews national television to help explain finance topics to the general public.

Maybe GP meant Americans alive today. Volcker is probably memorable mostly to an age bracket swiftly shrinking.

There's a fairly obscure T-shirt designed by FT Alphaville that just cryptically says "Allen, Ben, Janet, Jay" ;-)