To what extent it matters: I happen to have be quite familiar with Swedish copyright (apart from more recent changes made after the events in the article).
> but Swedish law looks a bit vague and it’s possible to assign ownership via implicit license.
It's quite clear on this part - economic rights and right to use can be assigned to others no problem like most places but creative ownership can never be renounced, explicitly or implicitly. Details below for the curious.
Copyright is divided into economic rights and moral rights/droit moral (ideell upphovsrätt).
For the most part, moral rights can not be transferred - any contract claiming to do is void. Part of moral rights is the right to attribution - this can be signed away, but only "under kind and art limited use of the work" (that is, a poem could have been explicitly signed away for e.g. use as ending in a PC game title, but I have a hard time seeing the verbiage by Carl at the time of the acquisition being valid). Also, even if there is a contract full permission of use and reassigning all economic rights, unless explicitly noted the creator would still have an enforceable perpetual right to attribution.
More interesting is the "right to respect". It gives the right of the creator to object to changes to the work, or publication in a context, which damages the artistic reputation of the creator, which according to the explanatory memorandum means "violation of the author's personality, as expressed in the work".[1]
The author may sign an agreement promising the purchaser of a work to not execute their rights towards them (like Carl's rejected contract attempted, and like OP is effectively doing further down in the post when they announce it to be public domain).
droit moral limitations/transfers need to have a limited timespan (non-perpetual) and explicit and limited scope in use.
Relevant section of the copyright law ("Rättsfall" links relevant cases[2]): https://lagen.nu/1960:729#P3S1
Not sure to what extent Swedish copyright was/is relevant for the situation in the article.
When learning this in uni, the Swedish way was presented as somewhat different in the amount of protection for the creator, the limited scope under which author rights can be restricted or transferred. Wikipedia tells me that most European countries apart from the UK have somewhat similar interpretations, though.
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[1]: For example if the ending would have had a completely different twist, and adjustments made to the poem that changes its message, that could also be an issue even if the use and rights transfer was otherwise agreed upon. To give you an idea of how far this can go, a film director was awarded damages on the grounds of TV channels making inappropriate cuts for commercials when airing their movies: https://lagen.nu/dom/nja/2008s309
[2]: Related to this case: a reporter writing a news article for their employer, a local newspaper. The next day the same article was republished in another publication owned by another company, with the permission of the employer. The court held that the employer had no rights to extend the economic license to another company, and the second paper had to pay the reporter. Seems pretty clear-cut that if Swedish law were to apply, Microsoft has 0 rights wrt the economic rights of the poem and would be liable to compensate OP for past unlicensed use, up until the point of the blog post where they renounce these rights moving forward. https://lagen.nu/dom/nja/1993s390