This distinction makes sense. The point is that it should be possible to delete whatever is in parentheses and the sentence should still make sense. The parenthetical part is just extra information to allow someone to find the source.
The first person to prove this was Smith (2005)
passes the test: the main sentence is "The first person to prove ...
Function of OP's writing is "undergraduate essay" which I assume means that the grading criteria don't include correct referencing.
In that case there are two approaches:
most likely any way you cite/reference is OK. Nobody will likely check the quote, and if you quote directly then they will be able to find it using text search
when in doubt ask ...
Whether you like it or not, the APA style requires you to write "n.d." instead of the missing date.
However, when you examine the file properties of the referenced document, you can see that it was compiled on 13 September 2019. I couldn't find an authoritative source on this, but it might be fair to date the reference accordingly.
How your ...