Take that with a pinch of salt, what matters is the state of charge of the actual battery cell, which is not what the car reports because manufacturers reserve some of the battery capacity. The GTE has a significant battery buffer built into it already - the Passat only lets you use ~10kWh of a ~13kWh battery (I think the mk7.5 Golf is the same). I forget exactly how the buffer is distributed, I think it's something like the bottom 10% and top 20% are reserved, so when you do a full charge, you're getting something similar to a 20-80% charge anyway. So if you limit yourself to only using the middle 60% of the usable range, you're really only using the middle 45%(...? morning maths...) of the actual battery. Sure, your battery would probably last forever if you did this, but that's because you'd be barely using it.I understand with a full EV, its best to stick to something like 20-80%, but the battery on a PHEV is a lot smaller and will inevitably go through far more charges.
PHEVs tend to have much bigger battery buffers than full EVs - partly because their batteries get cycled an awful lot more, and partly because they don't need the full battery range to keep mobile. Fundamentally, the same recommendations apply to the battery regardless of what kind of vehicle it is in (VW physics is the same as Nissan physics is the same as Hyundai physics...), but how much it matters in a specific vehicle depends on how the manufacturer have chosen to use the battery. Compare the GTE usable battery to something like an ID3, where you can use something like 90-95% of the actual battery capacity - that's where you need to have a think about whether you need a full charge or not.
Also, different vehicles will have slightly different battery chemistries, which will have slightly different recommendations. There are no hard and fast rules here.