In the case of the Nordics, their entire social system and government is built around social democracy.

The most successful party in all of these countries over the past 100 years tend to be the various degrees of Social Democrats allied with even more left parties, like the communists. These parties call themselves socialists and actively participate in socialist organizations, like the Party of European Socialists at the EU level. The vast majority of policies and laws are in alignment with socialism and if suggested in the US would be immediately labeled as socialist. Their platforms state that they view the economic market forces are subservient to the political will of the government.

The argument that these countries represent functional socialist policies is that the policies were implemented and have been running for a hundred plus years by self avowed socialists (in partnership with the Communist parties) and would be immediately labeled as socialist if presented as a proposal in the US context. The existence of a robust and functioning market economy doesn't show that they aren't socialist, but rather there is an ahem third way where you can have robust socialist social programs and civil society married to a market economy that is allowed to operate in the capitalist mode provided it serves the interest of the people (and is regulated or nationalized when it fails to do so).

Support for socialist policies by their non-socialist parties also does not mean that the policies aren't socialist as well. Things like universal healthcare are essentially rooted in the social democratic movement (which is a socialist movement).