I have an all AirPort Extreme network at home with multiple units. My friend just got a brand new WiFi 6 modem from Bell Canada for his Gigabit internet access. I get faster speeds on my AirPort Extreme 802.11ac than he does. He doesn't have any WiFi 6 equipment, so it's a pointless feature for him anyway. We both have 802.11ac Macs and iDevices and I get ~500 Mbps download at 25 feet away. No complaints, and the Apple hardware is uber stable, more stable than many other non-Apple solutions unless you go with higher end $$$ stuff. In fact, this latter point is why I stuck with Apple gear. In an Apple environment, it works seamlessly and it's easy to administer.
BTW, I get automatic handoff between AirPorts in this house with my Mac and iOS/iPadOS gear, as I move around the house. The downside is that this automatic handoff does not work properly with non-Apple WiFi clients, but I don't care because all my non-Apple clients are stationary anyway (eg. thermostat, TVs, etc.).
The other downside is this setup doesn't support features like QoS, but it sounds like that isn't an issue for the OP's gift recipient.
Typically, ISP service is slower than most AC WiFi. Thus, the only possible gain is file transfer speeds between hosts on the same network which might be faster with WiFi 6 devices. But if the ISP link is 100Mbps or less, nothing you do on the router or access points will speed up internet service.
That may be true, but Gigabit service is extremely common in many countries now. I'm on Rogers Gigabit cable, my friend I mentioned above is on Bell Gigabit fibre. Mind you I'm downgrading to 500 Mbps cable service next week, to save ten bucks a month.
I think the bigger issue is most of us don't own many (if any) WiFi 6 clients, and even if we do, sometimes the equipment we have doesn't max out WiFi 6's capabilities.