By RAW though a Parrying Dagger is not a Dagger and cannot be used as a sorcerer implement.
I see what you're saying - and yes, there is no rule that defines a dagger weapon group, and no rule stating the parrying dagger is part of that group. I think the intent is indeed for the sorcerer's implements not to include the parrying dagger.
However, the reference to daggers isn't somehow magically precisely defined. It just says "daggers" - no specification of type. The game being described in plain English, it's not much of a stretch to interpret a parrying dagger as being a type of dagger. I think it's unwise not to accept the inherent fuzziness is such descriptions. Assuming the text is 100% rigorously defined requires all sorts of disambiguation which may cause disagreements, and the colloquial nature of the majority of the rules text underlines the fact that it's written in plain prose, not some kind of specification language.
From the perspective of the rules-keywords, it may seem that there is only one dagger, but conversely it's hard to argue that the parrying dagger is definitely not a type of dagger. And it's not without precedent that an implement category might be refering to more than one specific weapon; witness the swordmage.
It's
certainly easy to see the argument that the term "spears" in the monk description refers to any spear, not just "the" spear - without checking the write-up, I can't say whether additional clarification and specification is given.
Edit: On review, I think I somewhat overstated my case. I'm pretty sure the intent isn't to include parrying daggers; the fact that the rules are fuzzy is true, but it's just not that relevant here - where weapon groups are always described using the obvious keyword-based mechanic. It's less clearcut for groups that share a name with a single weapon, unfortunately.