I agree that EK's should be able to use their bonded weapon as a spell focus, but it would still apply to their wizard spells.I just wonder how many DMs would enforce this? It just seems so nitpicky. Especially given the leeway clerics and paladins are given with shields. It seems punative to enforce this on the EK when it's kind of their schtick to cast spells while wielding shields and armour. Because, even without taking shillelagh, an EK can't wield a sword and shield and cast the spell shield. They, literally, need to wield a staff.
It would have been nice if they had a power that their bonded weapon could work as a spellcasting focus. Which still wouldn't solve the shillelagh issue but at least they could cast their own spells.
Clerics and paladins have leeway because holy symbols don't need to be held, only worn.
Holy Symbols
Symbol Weight Cost Amulet (worn or held) 1 lb. 5 GP Emblem (borne on fabric or a Shield) — 5 GP Reliquary (held) 2 lb. 5 GP
and
to use a Spellcasting Focus, you must hold it unless its description says otherwise
That's an advantage to playing a cleric or paladin. They only need to wear their holy symbol depending on the type, but that doesn't necessarily give them a free hand if they might need one either. A cleric wearing a shield also can't cast Shillelagh that way unless it's become a cleric spell for them somehow.
A spell component pouch also requires a free hand.
The easiest solution for EK's is recognizing and accepting that shields or two weapons can become problematic and build for a two-handed weapon. Otherwise, a person needs to figure out work arounds like the warlock splash.
As for what a DM may or may not enforce, that's speculative. I don't think we can discuss how rules interact if we're speculating on house rulings outside of the printed material.