No "metric" units means they are derived from the meter. Originally, most units in the SI systems were derived from the meter.
(But not really; that was originally considered as a way to standardize the meter, IIRC, but the period of a pendulum varies too much even over the area of France to be used as a concrete standard. But the relationship is remarkably close for a coincidence, like the way a rod is almost exactly five meters.)
The second is defined by taking the fixed numerical value of the cesium
frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of
the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which
is equal to s−1.
The SI units are supposed to be related like that.
Seconds are also part of the metric system, but one of the few not based on decimal/base-10.
Again, I’m just speculating that the author used “metric” because it often represents decimal/base-10 measurements. Not really arguing whether they were technically correct in doing so.
https://www.bipm.org/en/si-base-units/kelvin
Anyway, TIL.
For volume this is obvious nonsense since the metric system expresses volumes in … cubic meter! And even weight, a kilogram is “the weight of a liter of water” (that is a thousands of a cubic meter of water).
Maybe we need a cron for an hour or a day? 1,000 crons for an hour sounds better than millicrons by breaking up a day if it was 1 cron.
"Centi" is an SI prefix just as much as "kilo" is, and has been part of the metric system since 1795 just like "kilo". I don't see how it's "not in the spirit" of the metric system. It also fits in neatly in that 1cm^3 of water is roughly 1 gram (the original provisional definition of gram was 1cm^3 of water at the melting point of ice; the current definition is more precise), and so 10cm^3 of water is roughly 1 liter.
The SI system has prefixes going up and down one power of 10 up to 10^3 and down to 10^-3, and then in steps of 10^3. They're all equally part of the system; some are just more common in some contexts that others (e.g. we use hectograms but rarely hectolitres, and decilitres but rarely decimetres, and centimetres and centilitres but rarely centigrams) depending on what happens to be convenient.
E.g. kilo(10^3), hecto(10^2), deca(10^1), deci(10^-1), centi(10^-2), milli(10^3), but then mega (10^6) and micro(10^-6) are the next steps.