[Quoth]

As expected, a zip-lock bag works perfectly (possibly BETTER) on the capacitive Libra, but only taps (and not always) on the original Aura H2O.

The HZO lacquer can't be trusted. Also if the USB connector is wet you may get electrochemical corrosion of the body of a plug. Long term you could get PCB corrosion via the USB connector.
Gold doesn't corrode at all. But in any damp situation every other non-noble metal will have accelerated corrosion. That's why gold plated audio connectors are madness on any regular nickel or tin or chromed socket on equipment, or even near a damp connector. The rule is only gold to gold.
Also how thick is the gold plating on the USB connector? Most consumer connectors have too thin. Ironically impure gold lasts better. Purest gold is softest. Electrum is a pretty ancient alloy of gold, silver and copper. Still used as it's much harder. Alloys with other noble metals are good too.

While the rubber bung on the original H2O could wear out or break off, a sealed case is a far better idea than a coated PCB. How reliable will the coating be for display connector interfaces?

A screw on watch back with an inset neoprene ring is reliable. The original H2O was not. It relied on double sided sticky tape in one piece to stop water getting in at the screen (which meant a gap for dust) and at the outer edge of the bezel which wasn't inset to case so it could catch. Some have been poorly assembled and had a gap on the outer edge.

Add a zip bag if on the beach, pool, camping or in the bath. Even if the HZO coating works today, it will fail. The exposed USB and gap at the buttons on the Libra is mad. Blow in at the USB and feel air come out at the buttons.

I didn't trust the original Aura H2O which cost me €220 at the beginning; it's certainly not trustworthy now. I'll not trust the Kobo Libra H2O either. Less so, as it's impossible to visually gauge any degradation. They (HZO call it "nanotechnology". So is two part polyurethane varnish*, which will waterproof a circuit board). Independent Figures for how good HZO is on something with connectors and degradation with time that are independent tests? The awards received say nothing about the goodness of the technology, but about marketing. This is an 80 year old concept.

(*The Polyurethane eventually develops cracks. Wax was used 1830s to 1950s, think something closer to Vaseline than candle wax so it does crack. Water amazing does dissolve in it (and in paraffin, diesel, jet fuel etc) so after 8 to 12 years protected parts and wiring start to corrode. In the 1940s military gear switched to a mix of hermetic sealing and special varnishes.)

I'd be very upset to lose €180 worth of Kobo Libra H2O due to a dunking.