Since a helical monopole is 90 degrees long at the design frequency,..

It may have ~ zero reactance, as does a linear ~ 90-degree monopole, but
the helix will not have the radiation resistance of the linear version, as
John Kraus pointed out in the linked page.

Radiation resistance is a function of the end-end length of the helix and
the frequency, whether the helix is self-resonant or not. The radiation
resistance of the resonant helix in Kraus' example is much lower than that
of a series-fed, 1/4-wave, linear monopole.