Liberalism has a couple of different definitions. The one you're thinking of is the one in US politics where "Liberal" is synonymous with "Left'. This isn't how it's being used here though.

Liberalism, as a broad ideological trend that came out of the enlightenment, contains within it, Conservatism. Conservatism was theorized by people like Edmund Burke who, seeing that the previous feudal hierarchy was dying off, sought to preserve it, at least as much as was possible, by accepting Liberal notions of property rights and capitalism.

So, instead of a social hierarchy being ordained by God, it's decided by the market, and social conflict is meditated through the liberal, Lockean, Republic.

So when we call Trump a liberal, we mean it in this broad sense. He's still a conservative, but conservatism is a subset of capital L Liberalism.

This is in contrast to Leftism, which also contains a lot of things within it, but breaks from a lot of the philosophical assumptions that undergird Liberalism.