It's just a measure of "1 in a thousand". Or put another way, the distance between the centres of two adjacent mildots is one thousandth of the distance of whatever you're aiming at.
Hence 36" at 1000 yds is a true mildot because 36" is one yard: 1 yard at 1000 yards.
Likewise 1 metre at 1000 metres is a true mildot.
Mildots apply to both metric and imperial measurements, or any form of measurements, but because metric uses multiples of 1000 it makes the calculations easier. 1 mildot is 40mm at 40m because there are 1000 mm in 1 metre.
If you work out what the actual angle of a mildot is, it is 0.0573 degrees. MOA (minutes of arc) are 1/60th of a degree so this works out at 3.44 MOA.
So 36" at 1000 yds is "true", 1m at 1000m is "true", and 3.44 MOA is also "true" (or 3.438 rounded to 3 places instead of 2).
A mildot is not exactly the same as a milliradian but I'm really really not ready to go into the difference!