He's wrong about one thing, though.
Using a ruler without an inch scale (although all my rulers have that), is not at all hard. You just hold a ruler to the chain, and check how far off 25.4 cm it is.
If it's up to 25.5, that's 1/25th of an inch. 25.6 is 1/13th of an inch.
1/13 and 1/16 are not too far apart. And 25.7 cm means 1/8th of an inch.
Easy!

My bike is almost brand new, so it has no chain wear to speak of (110 km so far).