He didn't have to hide his identity from a WSJ reporter because journalistic sources are protected by similar set of laws. To the readers of WSJ he was just a "confidential source".
Then the article says: "In March 2015, Tyler Shultz was contacted by a Journal reporter through the professional network LinkedIn."
Something doesn't match up here. I can only assume that his anonymous complaint did not reach the desired effect in a year, and he went via other channels, using his own renouned last name.
It would also seem rational for the investigative reporter to ping 20-30 LinkedIn contacts who listed Theranos as their employer in hopes that 2-3 would reply, so maybe he wasn't the only one the reporter contacted.
Was Tyler the only confidential source to talk to the WSJ? Probably not. because of that whole two sources rule good journalists follow.
Schultz Jr was contacted in LinkedIn by a WSJ reporter, he did not hide his identity because he likely (empirically) realized that his anonymous complaint would be ignored.