The truth is not a defense against libel laws in all countries. Depending on where this is the poster could be out a lot of money just for naming names. As such not naming names is the safe answer.
Even in the US where the truth is a defense, you still can be out a lot of lawyer fees because you can be sued for things you say and it can cost a lot of hours in court.
The author is located in Italy, where "it's the truth" is not an absolute defense against defamation like you say - basically, here, causing "reputational harm" is actually against the law, even if you are telling the truth. There are a few exceptions like social interest which may apply, but it is a dangerous game to play because you need to prove that to the courts, as opposed to just proving what you wrote is what actually happened.
Plus, any court proceedings in Italy can routinely take decades, destroying one's life even if they are completely innocent, even if the complaint is trivial, even if the complainant is obviously malicious.
In the USA it used to be very rare for companies to directly mention competitors in ads. Products would be compared to "Brand X" or some other genericized name instead.
I think it still is somwhat rare. Why even let a potential customer know that a competitor exists?
a company with a history of threatening baseless lawsuits, combined with possible NDAs, or possible professional backlash when lawsuit-happy company threatens former employer. not worth it for a blog post.
Moral of the story is that going to open-source is only part of avoiding the traps that vendors set. You also have to trust the vendor you're working with and make sure that the contract isn't full of lawyer tricks.
Even in the US where the truth is a defense, you still can be out a lot of lawyer fees because you can be sued for things you say and it can cost a lot of hours in court.