Yes, of course we could call any threshold a “portal.” I was reserving the term for a very thick archway or tunnel that really says “You are leaving one place, and when you get to the other side of this tunnel, you will be in another place.”
The theme of the portal itself could be tied to the place you are going to (Main Street entrance tunnels, the Castle), or the place you are leaving (Frontierland entrances to GE).
But most of the other transitions across the DL resort are not handled via what I would call a portal. There may be signage on an arch, but it’s not a tunnel through time and space. (I don’t call AL, FL, or TL entrances portals. They are just entrances.)