###Is it technically correct...? No, of course not. It is neither technically, nor legally correct, for the same reason "fat free milk" (yes, this exists!) isn't, and cannot be.

Technically, milk is an exocrine secretion produced by mammals (in the tell-tale mammary gland) consisting of predominantly emulgated fat in watery solution and casein (protein), some sugars, and some salts. Anything not coming out of an animal's tit therefore is not "milk". Anything not containing a considerable amount of fat is not "milk".

Legally, there are, depending on where you live, more or less hefty regulations such as e.g. the FDA's in the USA, or about half a dozen EU laws in the EU which control pedantically every aspect of what may be called "milk" and "some type of milk" as well as "milk product", including microbiological and technical minimum stadards and minimum fractions in its composition. The word Milk is a protected term, which is basically something like a Trade Mark (similar to Champagne, Camembert, or Nürnberger).
Anything that did actually come out of an animal tit but underwent processing in excess of heating and segregation by skimming is not "milk", and depending on what was done and what composition it now has, it must bear a prefix (such as pasteurized, H-, fat-reduced, etc.).
Milk being sold without animal prefix is (at least in the EU) defined to be "cow milk". Where "cow" should more precisely be "cattle" (but isn't) as "cow" can, depending on the language chosen, be quite ambiguous species-wise. So, e.g. "goat milk" may not be sold as "milk", although it is milk.

###Why was it called...? So why did and do people call white liquids (including the sap that comes out of dandelion) milk? Well for the same reason that diamonds or gems in general (including amber) are stones.

White liquids are white liquids, and milk is a white liquid. 99% of all people aren't biologists or lawyers or overly bright or educated (especially not in the middle ages), and few would care anyway, for that matter. If something looks like a duck, it is called duck.

Is it technically correct...?

No, of course not. It is neither technically, nor legally correct, for the same reason "fat free milk" (yes, this exists!) isn't, and cannot be.

Technically, milk is an exocrine secretion produced by mammals (in the tell-tale mammary gland) consisting of predominantly emulgated fat in watery solution and casein (protein), some sugars, and some salts. Anything not coming out of an animal's tit therefore is not "milk". Anything not containing a considerable amount of fat is not "milk".

Legally, there are, depending on where you live, more or less hefty regulations such as e.g. the FDA's in the USA, or about half a dozen EU laws in the EU which control pedantically every aspect of what may be called "milk" and "some type of milk" as well as "milk product", including microbiological and technical minimum stadards and minimum fractions in its composition. The word Milk is a protected term, which is basically something like a Trade Mark (similar to Champagne, Camembert, or Nürnberger).
Anything that did actually come out of an animal tit but underwent processing in excess of heating and segregation by skimming is not "milk", and depending on what was done and what composition it now has, it must bear a prefix (such as pasteurized, H-, fat-reduced, etc.).
Milk being sold without animal prefix is (at least in the EU) defined to be "cow milk". Where "cow" should more precisely be "cattle" (but isn't) as "cow" can, depending on the language chosen, be quite ambiguous species-wise. So, e.g. "goat milk" may not be sold as "milk", although it is milk.

Why was it called...?

So why did and do people call white liquids (including the sap that comes out of dandelion) milk? Well for the same reason that diamonds or gems in general (including amber) are stones.

White liquids are white liquids, and milk is a white liquid. 99% of all people aren't biologists or lawyers or overly bright or educated (especially not in the middle ages), and few would care anyway, for that matter. If something looks like a duck, it is called duck.