Ownership is a social construct like any other. Limiting your intepretation of class to ones defined by ownership makes the analysis pointlessly inapplicable when looking at societies where ownership plays less of a role. Feudal society didn't just have lords and serfs, it also had peasants and free cities and church land that don't fit the "feudalism is defined by owning the land and the serfs on it" schema. Theocracy, bureaucracy, caste systems, white supremacy, patriarchy, and anarchy all benefit from a class-based lens.
Marx focused on ownership as the primary class divide because that's how capitalism works (especially in the 19th century before social liberalism muddied the waters through pension funds, unemployment benefits, small business loans, etc.), but any conflict of interests between groups of people seeking to achieve economic prosperity will do. For example, migrant workers are actively imported under liberalism to divide the working class, which was so succesful not just because of racism but because of genuinely different economic incentives between those with citizenship privileges and those who are floundering in a hostile legal system but for who what little they get is more than they could have gotten back in the imperial periphery. In essence, citizenship becomes the division between two distinct working classes.
And here we see how solidarity is a matter of interpretation. Right-wing citizen workers attempt solidarity with capitalists by invoking their shared citizenship while left-wing citizen workers attempt solidarity with the migrants by invoking their shared worker status. These are different strategies that are both within range of what the material conditions make plausible. One hopes to reinforce imperial oppression, creating the material conditions that can be leveraged into greater citizenship privileges as traitors to the international working class; the other hopes to leverage the power of a united labor front grants either to demand more privileges at the negotiating table (socialist reformers) or to have a revolution whose reorganisation leaves them better off than before.
I wish I could say for certain that being traitors to the international working class is a foolishly bad bet. But it worked okay for western workers in the past century. I truly can't say if a median income middle aged white male American would expect less prosperity under Trump than in the conflicting mess that would be an American leftist revolution.
Those in power have similar choices between mutually contradictory strategies, reshaping material conditions in different uncertain ways. In 2015-2024, the USAmerican capital class was divided between going full hog dictatorial fascism and trying to reform through the slow decline of their empire. Neither is objectively better for the US capital class; they both have different risk profiles, and fascism is a big enough swing that it can't be hedged against very well. If the US collapses, billionaires may well end up being hunted down and killed, which is worse for them than becoming multi-millionaires in a world where China is the dominant nation.