Supported hardware doesn't matter because they're comparing the compiled binary size, not source code size. The u-boot binary you'd compile would also only have the stuff relevant to that particular hardware compiled-in.
If you don't need the other features of u-boot that this doesn't have, it makes sense to count the lower binary size and runtime memory usage as an advantage.
That said, they compared it to "an example U-boot binary", which sounds like they probably didn't tweak the bajillion config options u-boot has to produce one with an equivalent feature set to theirs, which would've been a fairer comparison for sure.
If you don't need the other features of u-boot that this doesn't have, it makes sense to count the lower binary size and runtime memory usage as an advantage.
That said, they compared it to "an example U-boot binary", which sounds like they probably didn't tweak the bajillion config options u-boot has to produce one with an equivalent feature set to theirs, which would've been a fairer comparison for sure.