I think it's worth remembering the Clarity didn't come out in 2018. The fuel-cell version was a 2017 model, released December 2016. So, it wouldn't be Honda refreshing a one year old car (that's never happened, AFAIK, except the disastrous 2012 Civic), it'd be them refreshing a two year old car. But, that's still not very common. The refreshes usually come after 3+ years.
Also, they usually launch the refreshes in September, while unchanged models don't switch model year until the end of the year. E.g. there are almost three thousand 2019 Civics on lots across the country right now, but zero 2019 Accords, because the Civic is a refresh next year while the Accord is unchanged.
I think the only hope of a 2019 Clarity refresh is if Honda doesn't follow their typical m.o. with them due to them being low-volume and such.