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?
Best Answer
Yes, the Eldritch Knight can use a Ruby-ed Weapon as a focus
You don't need a special feature to use a spellcasting focus. You normally need a feature which lets you use X item type (arcane or druidic focus, musical instrument, tools, sword, etc.) as a spellcasting focus. The ruby comes with its own feature letting you use it.
The general rule which covers the use of spellcasting focuses are in the Component Rules of spellcasting
However, by default there isn't anything which is always a spellcasting focus. That gets defined by a feature (or subfeature of the Spellcasting feature) for the specific caster.
The Ruby would be one such feature (creating an additional, specific rule for the relevant character) which lets you use the specific weapon as a spellcasting focus. Perhaps of note, it is not the only feature which allows a weapon to be used as a spellcasting focus, see the College of Swords and their Bonus Proficiencies feature (also from XGtE).