As explained in How does an arcane focus work with an Eldritch Knight?, Eldritch Knights can’t use an arcane focus as a spellcasting focus.
Ruby of the War Mage (XGtE p. 138), however, states:
Etched with eldritch runes, this 1-inch-diameter ruby allows you to use a simple or martial weapon as a spellcasting focus for your spells. […]
This paragraph doesn’t mention anything about arcane foci or anything like that, but straight up says that you can use the weapon as a spellcasting focus; the only requirement being that you are a spellcaster (which is true for Eldritch Knights).
Can you use this magic item to circumvent the otherwise existent restriction to component pouches for Eldritch Knights? Or am I missing some rule that disallows this "trick" / workaround?