> 50% means you only get 1 tag bit.

That's enough to distinguish between "unboxed float" and "something else", where the latter can have additional tag bits.

> [64-bit] pointers are only 48 bits and you have 53 bits of NaN payload.

The paper specifically talks about support for "high memory addresses that do not fit in 48 bits". If you don't have to handle those high addresses, I don't think this approach has any benefits compared to NaN-boxing.

Of note is that even if you have some massive ≥2^48 data sources, you could still quite likely get away with having NaN-boxed pointers to within the low-size heap, with an extra indirection for massive data. This only would break apart if you managed to reach around 2^45 distinct referenceable objects, which you probably shouldn't ever have (esp. in a GCd language).