What else would it be? The whole point is people seem to think way more people identify as socially liberal/fiscal conservative than actually exist. Every time there is third party talk, there is always a set of people who come out of the woodwork insisting such a party should exist (claiming a silent majority of sorts), but there seemingly isn't a constituency for it at all. Even the third party voters in the graph are pretty evenly distributed among the quadrants.
Question is what the social means. I typically think of socially conservative as traditional values, religion, etc and not necessarily but more often than not the desire to integrate those values into laws. That doesn’t need a big government imprint. Just a bunch of laws and modest law enforcement and administrative state. So the opposite of that would be more libertarianism, government stay out of social affairs. That is somewhat different than social liberalism that thinks the government must actively fund equity to the extent of significant financial intervention.
I think you are overanalyzing a sort of platitude that existed, and to be fair, its currency has waned since 2016. I'm not sure what "social liberal" ever meant other than people not down with religiously backed agenda for morality. I do think its interesting that America is at its least religious ever while a seemingly religious agenda is riding high with Republicans. Its clear that 'social conservativism' however you wish to define it can be secular in nature, as plenty of people who back and agitate for Republican anti-drag or anti-gay laws go to church as often as I do.
Well yeah, the whole picture is. But libertarians, in many cases, would consider themselves socially liberal. They don't agree with government funded social change, but they support individual liberty to high degree. I don't put a lot of stock in these labels honestly. But progressive would probably more specific than liberal for people who strongly support funding government programs to improve outcomes.
This is true. Libertarian types are highly visible on college campuses and on message boards, but much less numerous as a percentage of the electorate. And there is also a large number of people who strongly support values issues, but who are more or less indifferent to corporate tax rates or privatizing social security, even if they themselves don't like paying taxes.