Immigration is a huge elephant in the room here. Sweden has admitted an enormous number of immigrants from the Middle East and Africa. If you don't correct for that, your results will be meaningless.
I had plenty of problem students, and they were all Swedish. The issues have more to do with teachers not being allowed to tell them that they're wrong, to the point where they develop glass jaws and/or a complete lack of respect for their teachers and their own education. They just don't care, because nobody sets boundaries for them or tells them to push themselves to get better at something!
I was almost fired from a university for telling my students that the majority of them did not bother to write down their names, student number, use paragraphs, capital letters at the start of a sentence, or even something as basic as putting dots between sentences in their weekly one-page reports. That they all knew that they wouldn't have made it past the first year of high-school with that kind of attitude. That I could not in good faith accept this from adults who were two years away of having to write their own CV letters. And that therefore any report that did not meet these basic requirements would immediately be sent back with a note "format it properly" until it did (note: I would not even fail them - they would still be graded after the corrections).
This then resulted in a complaint to the higher-ups depicting me as some raging lunatic, and the higher-ups took the side of the complaining students with extreme prejudice, resulting in a near-firing.
If anything, the immigrant students had more fighting spirit than the locals, possibly because they're not pampered.
Something like 20% of Sweden's population is of foreign origin. If the paper doesn't account for this, it's got a problem. The performance of American students in PISA or TMSS by racial breakdown has been well-documented -- blacks and Hispanics drag the average down more than Asians drag it up. And you can find that many comparisons between American states are just the result of Simpson's paradox. So that's something you need to check for when looking at Sweden's performance.
(In the paper you can find good evidence that a big portion of the deterioration is not caused by ethnic makeup. For example, advanced proficiency dropping from 12% to 3% on page 9. But it's not so clear about changes to performance at the bottom end.)
Now, I do wish to give the GP the benefit of doubt, but the expression is problematic regardless of intent. My comment was more of a response that implied dog-whistle. Admittedly, it was a bit reactionary because of my own personal experiences surrounding these issues, and the ease of which I have seen people blame things on foreigners without even a hint of self-reflection. But as you mention: the paper suggests these issues are not caused by immigration.
He told me how tough it was for him to be in front of people who he felt were bullying him to discover afterward that he was, to take his words, a mimosa (a flower which crumples once touched).
He said that thanks God he had two brothers and he quickly recovered because they used to fight (like brothers do). He blamed, like you, the pampered way of life he had. A good life, just not ready for non-Sweden life.
I remember an article about Chinese students who had similar problems when they had to face student life, coming from one child families where they were demi gods to their parents.
I have a feeling that Swedish culture is a bit too conflict-avoiding in this regard, and not very good at distinguishing the two.
(To be clear: I was not ranting Linux Torvalds style)