I've had the experience several times where extensive, detailed emails would always meet the same reply: "Thanks. Let's talk about it in the morning." These people are wise to the game, and they rely heavily on the imperfect nature of human memory; memorialization in the written record must be approached very carefully.
The email chain that shows Employee A to be a hero equally shows Employee B as a victim in constant need of assistance, obstructing other work, taking valuable time from others. As such, "Bs" will avoid asking in a recorded form, avoid giving credit to helpers when writing about their work, and so forth. Even if they mean well and earnestly want to credit the many people who've helped, it's too easy for a malevolent actor to turn it against them, so after a few raw experiences, they learn to avoid it -- just like what happens in the article.
The only real answer to these questions is to have an informed, rigorous, and fair-minded chain of command that has experience and cares deeply for the company's long-term well-being, which is, needless to say, quite rare. It is also worth noting that such noble persons are likely hurting themselves by acting this way and making enemies for themselves, as people who truly didn't deserve promotions, etc., whine and search for flimsy excuses.
In my experience people also ask for help through private messenger discussions (slack/skype etc), which of course don't leave any "official" trail and are not supposed to. Of course, a not-so-bright manager could force his/her direct reports to only use "official" channels for work-related communications, but then people would be afraid/ashamed to ask "stupid questions" in public/through an official channel and then disaster happens (it's also my experience that lots of serious bugs have been caused by people afraid/ashamed to "just ask").
There's only one way to actually measure someone's output, and it's working close to the team and witnessing their actual productivity. Getting involved. 1-on-1s.
Of course the farther removed from this process the more politics come into play.
Will show you a hero six months after they tendered resignation.
If you are assisting, and this includes providing support for someone else's code, or doing "invisible" tasks (setting up a redis cluster or writing end-to-end tests), this often finds its way into one email group or another. In fact I remember the names of the "helpers" as you put it by the virtue of their emails and to that end I value them, just as much if not more, for keeping us shipshape.
IMHO, you should find these emails, and I am sure they exist already, and use it as your metric.
Scott Fitzgerald wrote “Show me a hero and I’ll write you a tragedy.” I say show me email chains, and I will show a hero.
Of course I am not stating the obvious fact - which is that your manager, if he or she is worth their salt, should totally value the role "helping" plays in making good software.