Beneficial ownership of companies is the primary distinction between owner and employee. When you have taxation that targets companies but impacts rank-and-file employees while essentially benefitting owners and executives is consider regressive by definition.
I don’t know how much inequality needs to be to “qualify” as regressive but it’s not inappropriate to assign it to working class employees vs multimillion dollar salaries and 20-30M dollar comp packages.
A regressive tax is simply a tax imposed in such a manner that the tax rate decreases as the amount subject to taxation increases. Regressive taxes affect people with lower incomes (and everyone has a comparatively low income vs executive and ownership classes these days) more severely than those with higher incomes because they are applied uniformly to all situations, regardless of the taxpayer.
I’m not perfect and I might be misapplying it somehow, but that’s my understanding.
I also agree it's really bad tax policy.
But later you seem to switch to definition of whether a tax is regressive by comparing owners vs employees but that seems only tangential to whether or not the tax is regressive, which every definition I've seen has more to do with income level relative to the general population, not the 0.1%.