That is definitely not true for the late medieval period. As you suggested it would be too heavy without much of a benefit anyway. An added layer of chain mal won‘t block what a well crafted plate armor can‘t.
It is supposed to block the blows that go between the plates, not reinforce the plate itself. A lighter alternative that developed later is patches of chainmail only between the plates.
Mail armour is a layer of protective clothing worn most commonly from the 9th to the 13th century, though it would continue to be worn under plate armour until the 15th century.
In the chapter “Late Middle Ages”:
In armoured techniques taught in the German school of swordsmanship, the attacker concentrates on these “weak spots”, resulting in a fighting style very different from unarmoured sword-fighting. Because of this weakness, most warriors wore a mail shirt (haubergeon or hauberk) beneath their plate armour (or coat-of-plates). Later, full mail shirts were replaced with mail patches, called gussets, which were sewn onto a gambeson or arming jacket.
The German school of swordsmanship is more of a guide to fencing in south Germany than warfare across Europe. Reading up on it, there‘s also no mention of chain mail on that Wiki page. I would like to read up on an actual source of that because by the looks of it the author could’ve just come to their own conclusions here.
In Europe, use of mail hauberks continued up through the 14th century, when plate armor began to supplant it. Some knights continued to wear chain hauberks, however, underneath plate armor.[13] It remained in usage until the Renaissance.[10]
Milanese armour was characterised by its round, bulky shape and asymmetrical construction. A full suit of chain mail was also usually worn underneath the Milanese armour. In Germany, the Gothic armour was predominant, as well as later on the cuirass. The Gothic armour was characterised by its slimmer and more delicate construction. In addition, a complete chain mail shirt was rarely worn under the Gothic armour; more often, chain mail pieces were attached to the doublet in such a way that they covered the weak points of the armour.
That is definitely not true for the late medieval period. As you suggested it would be too heavy without much of a benefit anyway. An added layer of chain mal won‘t block what a well crafted plate armor can‘t.
It is supposed to block the blows that go between the plates, not reinforce the plate itself. A lighter alternative that developed later is patches of chainmail only between the plates.
In the chapter “Late Middle Ages”:
https://en.wikipedia.org/wiki/Plate_armour
The German school of swordsmanship is more of a guide to fencing in south Germany than warfare across Europe. Reading up on it, there‘s also no mention of chain mail on that Wiki page. I would like to read up on an actual source of that because by the looks of it the author could’ve just come to their own conclusions here.
https://en.wikipedia.org/wiki/Hauberk
Translated from german with DeepL:
https://de.wikipedia.org/wiki/Rüstung_(Schutzkleidung)
Here is an example picture of a Milanese Armour: https://en.m.wikipedia.org/wiki/Giano_II_di_Campofregoso#/media/File%3AHJRK_A_11_-_Armour_of_Giano_II_di_Campofregoso.jpg
Love it when you provide sources.