The README for hbase characterizes it differently:
hbase is a collection of programs that complements sbase
and ubase. It's meant to be a temporary project that
will shrink and die once sbase and ubase gets
implementations of most of the programs included. hbase
mostly contains programs taken from the Heirloom project,
but also has other programs, such as patch taken from
FreeBSD and mk taken from plan9port.
So it's not a rewrite, but rather a temporary code dump for "miscellaneous utilities". Skimming at the code confirms this.