Is an IP68 rating sufficient to protect a phone during a 12 hour ride in heavy rain?
It should be.  That's how I navigate on all my rides, wet or otherwise (phone on the bars).  I've done this for a few years, but not completely without incident.
Some tips:

Make sure all port covers are tightly closed; I even grease mine very lightly.
Consider how you will charge if necessary (I might otherwise plug in to a battery pack or my dynamo while going along)
It's possible for phones to leak after taking damage, even seemingly cosmetic damage.  My last phone did this after coming off the bars one too many times*.  I dried it out successfully a couple of times but then needed to replace the screen.  It's now my backup phone and not waterproof.
If it fails, how bad is it? Will you be lost in the cold and wet, down a large sum of money for a phone, or will you be able to ride home on known roads and repair it?  Should you have a backup map/route sheet?
Touch screens don't respond well when wet - will you need to touch the screen while riding?  I made a little windshield that keeps the rain off while I'm moving, but on steep hills or in stop-start traffic it gets annoying.
If you're considering putting it in a waterproof pouch for further protection, I've found they aren't all very waterproof, and anyway the clear plastic fogs to the point you can't read the screen if you let in even the slightest moisture (the window is colder than the phone).

BTW Garmin only claim IPx7 for their dedicated devices, so it's up to how well you trust the manufacturer and whether the seals have suffered over time.

* It's too heavy for most phone mounts and tends to find the weak spots and snap them, too fat for others so doesn't clamp securely.  The screen turned out to have slightly come away from the shell after the last impact with the ground at 35km/h.  It already looked battered.  The replacement has a lanyard clipped to the bars