The primary difference is the marginal cost of an additional customer. The definition of a tech company (at least for investors) isn't really about technology, it's about a near $0 marginal cost of adding an additional customer.
A great example of this is Facebook. For someone to sign up and start using Facebook, it really doesn't cost Facebook anything. Facebook probably only thinks about scaling in terms of orders of magnitudes.
Uber/AirBnb and other double-sided marketplaces debatably meet this definition. For these types of companies, the marginal cost of a new customer is $0. But these types of companies also have to onboard providers. Much of this is onboarding is automated but it can still require human intervention. Still, it's very cheap to onboard, there's not really any sort of capital expenditure that goes into scaling the provider side.
WeWork obviously has a giant marginal customer cost. This is the fundamental criticism of their business model. Lots of successful businesses like Amazon can do this but it appears like WeWork doesn't have any sort of scales of economy. For Amazon, you can put more products in the same warehouse, you can build warehouses closer to customers for faster shipping. WeWork pretty much can just open new locations with the same model.
Note that WeWork is trying to expand into other markets like gyms to get some of these benefits but whether or not this is a scalable business model is debatable. So far, it hasn't worked and there's not a great indication it will.
You mention that WeWork could change its business model to get better scalability. Of course they could change it but so could a well-funded startup. So could AirBnb. Frankly, it's a hypothetical that isn't proven out. Basing a valuation on a what-if that isn't a natural extension of what they're already doing is extremely risky. To do what you're saying, WeWork would basically have to start from 0, maybe with some expertise about what customers want. AirBnb could immediately launch the same service with 10,000 locations by leveraging their existing hosts.
How they got a $50B valuation seems deceitful and ridiculous. They conflate using technology and modern appeal with technology business models. Currently, WeWork does not have any business model that could be considered $0 margin customer cost. Maybe one day they could find one. Probably not.