BroadPwn Exploit

Yes, your iMac is vulnerable to the BroadPwn exploit.
That security update did not address the BroadPwn exploit.
There is currently no "fix" for that specific exploit for Mac OS X 10.11 or earlier, other than turning off Wifi.

 

Any reason in particular you are sticking with Yosemite? I understand the "if it ain't broke" mentality but in my experience Yosemite was the worst performing OSX release the 2013 iMac had.

--- Post Merged, Jul 26, 2017 ---

BTW, do you have a source for this? Yosemite security update 2017-003 was released from Apple the same day they patched the exploit in the rest of their line up.

 

It's right in the security notes for the update. The wifi patch only applies to 10.12.6.

Notice how 10.10.5 and 10.11.6 are not mentioned in the "available for" section.
From https://support.apple.com/en-us/HT207922