There are three reasons to sue: (1) as a deterrent, (2) to get an injunction and/or (3) to recover financial damages.
The former employee appears broke, so (3) goes out of the window. (2) is a possibility, but mitigated given he already shared the data. As for (1), unemployment + threatened criminal charges would do as much work.
Now let's look at the downsides. Tesla has Streisand effected this former employee's claims. The lawsuit will bankrupt the former employee if he can't show whistleblower status. That incentivises him to double down. At the same time, the public attention incentivises the SEC to look closer. Even without any wrongdoing, that attention is time consuming and costly.
If all this employee did was share with journalists and the SEC, the lawsuit could become a PR problem. Even if Tesla prevails, it will have dragged itself into an expensive distraction.
Assuming those charges are true, why would it be an overreaction to sue the person who did it!? That to me is the only valid reaction. This person is accused of knowingly purposefully trying to harm Tesla and help competition.
The only way this would be an overreaction is if Tesla really is guilty of doing illegal things. And in that case going to a DA would be the worst idea.