It's an old concept. Some references that are very old and possibly trace to their respective first editions...
In an old Kodak Data Book, Fifth Edition, 1956 (1947 also 1941, 1943, 1944, 1945) I found "Kodak Neutral Test Card: An accessory designed primarily for use in color photography, but quite useful in determining exposure for copying and other photographic applications. The gray side of the card has 18% reflectance; white side, 90%."
Interesting note: When you use the 90% card you are told to divide your EI by 5. (So EI 400 becomes EI 80.) 90 divided by 5 is 18. Any relation?
1970 (editions 1930, 1937, 1940, 1942, 1944, 1946, 1947, 1954, 1970) LP Clerk's Photography Theory and Practice section 475 Key Tone and Selective Readings. "Exposure errors introduced by subjects of abnormal brightness distribution can be avoided by pointing the meter at an artificial constant tone temporarily incorporated into the scene... If this tone is a medium gray of about 20% reflectivity, the meter reading obtained can be used as if it were an average reflected light reading of an ideal subject.
Wow - 20%, first time I saw that figure.
Now I wonder, with recent advice to take the 18% reading and open up.
And 20% dating to very long ago.
I wonder if the 18% was ideal when there was a safety factor in the ASA ratings of film.
Oh and a reading from spotmeter mode on 18% gray card (0.78 reflective density) and a reading from the incident mode on my meter agreed perfectly this afternoon.
Without stating the "K" of the meter, you could conceivably point it at just about any value reflectance card and get a correct average exposure.