When you say pre-discretized, I think of binning, is that what you mean?

Exactly. Instead of requiring the floating point values (4 bytes) and possibly an index (also 4 bytes) for each numerical values, a pre-discretized numerical value requires 2 bytes.
However, like for the boolean features (1 byte per value), the pre-discretized numericals are not yet available in the TF-DF wrapper (only Yggdrasil). I.e., this would be a feature request  .