If it's now perfectly normal to have only a single pilot self-monitoring during major changes of vertical flightpath, then safety has eroded considerably.
It's not difficult to organise your toilet breaks before TOD.
Which he did, he went to the loo right after reaching TOC. However, in europe there might be numerous level changes for many different reasons during the normal cruise portion of the flight. Some even having to do with how the company filed the flight plan. Had a flight on that routing lately and there were five planned cruise climbs/descends on the ATC flight plan.
As far as the Captain leaving the cockpit, I can certainly tell you that in the US, unless there was a significant ground hold delay after leaving the gate, it would be a small minority of flights of that length where a member of the crew would leave the cockpit. The fact that the Captain left the cockpit so soon after departure would be most unusual in the USA.
Which might have to do with the hassle of the two person rule, which still isn't mandatory in europe, although most airlines did implement it by now. I have since seen colleagues severely dehydrating as they want to reduce their loo breaks during flight. Something that is not really conductive to top physical and mental performance either.