Agnosticism is not about being uncertain, it's the position that the answer is unknowable or unknown - which is much stronger than merely being uncertain.
You have a point, but the person you're responding to makes a more important one - the colloquial definition of agnostic, in reference to religion, is a person who's not sure what they think about the existence of gods. When you call yourself an agnostic, that's what people typically understand by it.
If that's the definition then it's useless as a distinguishing label since everyone on either side of faith can't know. If anyone could know, faith and belief wouldn't be necessary.
Just because they wouldn't admit to it does not mean that the condition doesn't exist. And I have met Christians who have stated that they can't prove God one or another but that doesn't stop their belief.
Agnosticism has the greek root "gnosis", which means "knowledge". It's technically defined as a view that some things ("God" being a common, but not sole example) are unknowable. It's an epistemological concept.
Theism has the greek root "theos", meaning "God". Its primary concern is with the actual existence of God, so it's an ontological concept.
It's possible to think of agnosticism as side-stepping the theism debate by saying, "the nature of knowing matters more to me than the nature of being," but they're definitely compatible. You can say that the existence of God is unknowable (agnosticism,) but you believe that He's still out there (theism.)
Agnosticism is not about being uncertain, it's the position that the answer is unknowable or unknown - which is much stronger than merely being uncertain.