Agreed on the non-routine matters piece.Looks to be a rather specific situation of shares held in a 401(k). I think it's actually pretty unusual to be able to vote your 401(k) shares (or even hold individual stocks at all unless the Reddit poster actually worked for Tesla).
In a typical Fidelity brokerage account, they do not have the ability to vote your shares in the merger. They only have the "broker vote" power in routine matters (e.g., director elections, approval of auditors, etc). They don't have that power in non-routine matters like mergers or approval of additional share issuance.
One thing on 401(k)s just because I have done a lot of work in this area - it's fairly common for companies to now offer a self-directed brokerage option for the 401(k). In short, this means you can invest your 401(k) in individual stocks rather than the typical mutual funds. In my experience, most employees view this as "too risky" and very few take the opportunity to use the SDA option. I assume that's the situation they are referring to here with Fidelity.