Did they claim otherwise about their 2015 numbers anywhere? The only thing they mention is about Q1 2017. The 2017 numbers seemed to have improved drastically, something the article recognises (but the HN headline does not :S).
Read the report and you'll find out Tesla frequently has to update its numbers after the fact. So, yes Q1 2017 might look good NOW but in 3 months after injuries manifest and other claims come in (IBNR), it could turn to total shit. Tesla wants you to forget they frequently have to raise their numbers after the fact.
So, if history is any indicator, Tesla should shut its trap until it has a full year, after revisions, that show it's genuinely superior. Oh, I made myself laugh.
"Tesla did not dispute the numbers. “We may have had some challenges in the past as we were learning how to become a car company, but what matters is the future,” a company spokesman said. “With the changes we’ve made, we now have the lowest injury rate in the industry by far.”"