I agree that they cannot be combined and that there is precedent for this from prior rulings.
Foundation Strike converts a weapon qualifier attack to a spell qualifier attack, which would render it ineligible for Precise Blow. It also converts a “spell in memory” to a “spellstrike,” which renders it ineligible for Enhance Signature Spell or other abilities that can combine with a spell in memory.
Interestingly, I think this would also render it ineligible to be used with Riposting Blow, which allows a “weapon swing.”
If the intent is that RB could be used for a FS, but not Precise Blow, then I think some wording needs to be updated.