this isn't really an apple thing. thunderbolt requires the ability to use multiple displays, if you don't then you can't advertise thunderbolt.
since macbook airs don't have multi-display capability, you can't call it thunderbolt. you can call it usb4, or usb-c 40gbps, or whatever, but it's not technically thunderbolt - thunderbolt is the brand for the full-capability standard.
this is not something consumers normally bump up against since x86 laptops generally either have multi-display support or don't exist in a product tier where 40gbps is a thing. Stuff like chromebooks falls into the latter, too cheap to have 40gbps, and the entire x86 space is dominated by literally two companies both of which happen to support multi-display. but you'll probably see this show up more in ARM notebooks once other vendors have a shot at releasing processors besides just those 2.
like idk what you expect anyone to say here - usb standards are kind of a shitty free-for-all and always have been. usb gives you the freedom to make a confusing port that doesn't support everything. The simple fact of the matter is the base-tier Mx processors don't support multi-display without a software-driven dongle, that's all you need to know, it's otherwise a full-capability tb3 port, basically usb4 but without the baggage of being unable to connect to certain tb3 devices like usb4 sometimes does.
there is absolutely a justification for laptops like air to not have multi-display support. not everybody needs it, and those processors cross-over between the tablet and laptop spaces. it's a fancy tablet apu, regardless of how good it performs. the sin is not offering a M3 Pro variant on the 15" MBA, and I'd say offering the M3 on the MBPs at all. But it totally makes sense for ultramobility laptops to cut power and cost, and you'll almost definitely see ultrabooks start doing that as the wintel monopoly crumbles over the next year or two. That will be branded as USB4 and won't necessarily support all the capabilities of TB3... or be compatible with all legacy TB3 devices. USB standard sucks, or rather: USB Implementers Forum is responsive to their customers, who aren't you (at least not directly).