Absolutely, I know no-one on this thread has confirmed a solution using Safari v18, but I figured I'd report my own finding and see if anyone else had a different result.
I'm not inclined to switch my default browser just to resolve this, but I might consider using one of those browser-selector tools to see if that works in the meantime, and we'll see how long it takes for CheckPoint and Apple to find a workable solution. Even if Checkpoint were to give me more options for `idp_browser_mode` so I could select Edge / Chrome just for the VPN sign-on, that might be enough to get me through this blip.