I understand your point about fracturing politics on issues like this (we're still paying for "Brexit"), and that 10-20% is a fair chunk of voters, but even then, you've got to be really against it to throw your lot in with an extreme single-policy party. A minority of the minority will be disillusioned or angry about it.
Given that the vast balance of opinion wants to join, joining takes months and can be reversed, and both countries have general elections within the next 12 months, I think democracy will be just fine.
Here's the latest poll news from 2022-05-09. "Yes" at 76%. https://yle.fi/news/3-12437506
And additional interesting bits, both the president have backed joining, and Sweden is joining NATO too: "A possible Swedish application for Nato membership would raise backing in Finland to 83 percent.
A clear position by the Finnish president and the government backing membership raise support by around the same margin, to 82 percent."
Meaning the support to join is overwhelmingly in majority.
The most recent poll I've seen for Finland also has an even higher percentage of support for joining NATO now than your poll.
If you don't move quick, you won't move. This is the purpose of representative democracy - to allow trusted leaders to make decisions like this on your behalf.
In most representative democracies, treaties have to be approved after the fact but they certainly do not need to be pre-approved before being negotiated. This is not more undemocratic than a spur-of-the-moment referendum.
But, if we look at Brexit and how much Russia (apparently) corrupted that vote, I can’t say I’m shocked that either country would choose not to subject themselves to such a process.
In a non-democracy, the dictator decides whatever they what.
These are democracies, and so people elected their representatives, and the representatives decided to join NATO. I lived in Sweden for many years, and it seems like the general population indeed supports it. No law says you have to go a referendum, and it's definitely not against the constitution. The fact it was a quick decision is a actually great IMHO - people often see democracies as slow to respond, and here you have two countries moving a democratic process relatively quickly.
I am not making a comment that either is good or bad, just it seems that what “democracy” means tends to be pretty damned subjective.
It might also be that both sets do not overlap much. Nerds have always had strange opinions about what democracy is.
Rushed, decided behind closed doors, by a small clique of elites, without any semblance of democratic mandate, unconstitutional, no real public debate, no referendum, all in an atmosphere of McCarthy-like media frenzy. Any nuance is instantly brushed aside as Putin's propaganda
The polls put the support at ~52% in Sweden and ~61% in Finland. That means 48% and 39% of the people are neutral or against. This mass of people have no representation in mass media, and no major political party to back them. This void will be filled, probably by some outlier parties, possibly extremists. It's a recipe for a disaster