Top down can be isometric. Isometric means “same measurements.”

It refers to how the viewport is not like a camera above 3D terrain. Instead, you see each point below as though directly above. In other words, you can only ever see the top of an object regardless of whether it is centered or on the edge.

If it wasn’t isometric, you would see more of the sides of objects that are near the edges.

I've always assumed that isometric as used in describing videogames refers to the isometric projection, which is a very specific angle - or at least games that look approximately isometric. Age Of Empires, SimCity 2000, Constructor etc. Some of them were actually Dimetric or something else, but I'd never have referred to, say, SimCity 1 as an isometric game.

Are you possibly thinking of "orthographic"?