A mine shaft goes somewhere, namely to the ore which is to be extracted. Where does this thing go? Or where is it supposed to go?
I presume it is still deemed a tunnel due to the form of construction. The other end of shaft-mine “tunnels” leads to the minerals rather than back out into the open.
And frankly, the possibility exists that this one in Toronto was intended to be dug further, especially considering its location. It does not appear to be a bunker.