That is an overlimited definition. The definition is much broader than simply "the first cause". It is Huxley's generalized means of evaluating truth claims:
“In matters of the intellect, follow your reason as far as it will take you, without regard to any other considerations…do not pretend that conclusions are certain which are not demonstrated or demonstrable. That I take to be the agnostic faith, which if any man keep whole and undefiled, he shall not be ashamed to look the universe in the face…” (SOURCE)
As such it can, technically speaking, be applied to any claim.
"Huxley’s principle says that it is wrong to say that one knows or believes that a proposition is true without logically satisfactory evidence (Huxley 1884 and 1889)." (SOURCE)
Now, obviously, it has been and was initially posited in relation to theological matters but it appears to be a more generalized epistemic position to take. That in the absence of sufficient evidence one should defer to an agnostic position.
This is fair enough as it goes. But it is also why I have asked the board's various agnostics how they test truth claims in their lives. None so far have been willing to define the means by which they assess the validity of claims. My position is one that appreciates the agnostic position but also realizes that it is possible to test truth claims like verdicts in jury trials without perfect knowledge and still come to a claim which is not "agnostic".
The difference appears to lie in how we individually constrct our test statistic or criteria of assessing whether making a decision will commit an error such as a Type I (False Positive) error.
Whether one takes a view that knowledge must be perfect or they will defer to agnositicism or if one is, like me, willing to draw an inference from the testing of the hypothesis or claim is probably a matter of degree. I suspect that many agnostics reserve their agnosticism exclusively for matters of faith/theology rather than attempting to universalize the means of testing the veracity of claims in their everyday lives.
Even though it can be utilized in that manner.