I feel that a company should always give feedback to an employee who is let go. Whether it's a simple letter or an exit interview, individuals should have closure when they're leaving a company and it doesn't sound like the author received that feedback. Furthermore, if nobody - a manager, for example - explicitly says "your job performance is not up to par for your position and we need to improve that" before an employee is let go then that shows a lack of organizational maturity.
I think the gripes he had were somewhat fair; if a mid-level engineer is given a task outside his comfort zone and he communicates that he's having trouble then a good senior engineer should be able to provide design and implementation advice to the less skilled employee. In my opinion, that's a huge portion of being a senior engineer - if one employee is floundering in his technical task then you need to be there as a patient, understanding, and willing mentor. Heck, even senior engineers aren't immutable bastions of knowledge and should rely on their fellow senior engineers to be a mesh network of interlinking skill sets.
The email about the tweet seemed justified if a little petty. The author was expressing pride over contributing to the product his company was putting forth. While it could be argued that he should have linked to GitLab itself, it is a gray area and I feel that stark admonishment isn't a fair or correct response.
In the end, it's still a very public complaint and these articles are mostly frowned on. That being said, I appreciate that this Medium post exists. It's a point of view to take into consideration if I ever plan to interview at GitLab - even if the author is enacting the definition of 'burning bridges.' To conclude, whenever I read articles like this I always have to think "well, what other points of view are there to consider?"