I think it's an indication that Tesla were much better placed for enacting things like contactless handover and paperless sign-off than traditional dealerships. That combined with excellent (low) costs of leasing through NHS fleet solutions mean that it's an extremely attractive car to NHS workers, with immediate availability from stock (a number of people on Model 3 UK Facebook group ordered and took delivery in less than 1 week in April).
A colleague of mine has an Ioniq EV on order and it's been sat at the dealership for over 6 weeks now. I suspect that Hyundai are unable to reassign that car to a key worker requirement, whereas Tesla have stock and it's much easier for them to match stock to orders (far fewer colour/option variations).
Still, the model 3 has featured several times in the top 10 before (august '19 - it was 3rd).