As a founder, I honestly wouldn't have expected this to be legal. AFAIK, startups aren't allowed to co-mingle salary + healthcare benefits like that.
E.g. you're breaking the law if you reimburse someone's healthcare plan directly or allow them to expense a health expense, even if you classify that as additional income for them, as opposed to having a dedicated HSA account.
Do you have a source? I'd be super interested in how it works, and I definitely may be wrong on this (I hope I am).