  BH HMXB systems are likely the progenitors of the BH-BH mergers detected in
gravitational waves by LIGO/Virgo/KAGRA (LVK). Yet merging BHs reach higher
masses (~ $100 M_{\odot}$) than BHs in HMXBs ($\sim 20M_{\odot}$) and have
typically lower spins ($a_{\rm BH}\lesssim 0.25$ with a larger values tail)
than often claimed for BH in HMXBs ($a_{\rm BH}\gtrsim 0.9$). This could
suggest that these two classes of systems belong to different populations, but
we show here that this does not have to be the case. The difference in masses
is easily explained as the known HMXB-BHs are in galaxies with relatively high
metallicity, so their progenitor stars are subject to strong mass loss from
winds, leading to relatively low mass BH at core collapse. But LVK can also
detect BHs from low-metallicity galaxies that naturally produce more massive
stellar-origin BHs. However, a difference in spin is more difficult to explain.
Models with efficient angular momentum transport in stellar interiors produces
slowly spinning progenitors of both LVK and HMXB BHs. Known HMXBs have orbital
periods too long for efficient tidal spin-up, and are also unlikely to have
undergone significant accretion spin-up. We show that the derived value of the
BH spin depends strongly on how the HMXB accretion disc emission is modelled.
We argue that since Cyg X-1 is never observed to be in a soft spectral state,
the appropriate spectral models must take into account the Comptonization of
the disc photosphere. We show that such models {are consistent with low spin
values: $a_{\rm BH}\sim 0.1$.} This has been confirmed recently by other teams
for both Cyg X-1 and LMC X-1, and we show this is also the case for M33 X-7. We
conclude that all known HMXB BHs can have low spin, in accordance with the
results of stellar evolution models. Hence, current observations are consistent
with LVK BHs and HMXB BHs belonging to the same population.
