I don’t think this is true? My understanding, from my company which mandates sell to cover on vest, is that the cost basis is the price on the day of vest. The shares that are sold to cover are sold at the same price so there are no gains or losses and the amount that is sold is (in theory, if you’ve set it up this way) equal to your marginal tax rate so you end up paying around the correct amount to not have a huge tax bill.
If you set your sell to cover % very low, then yes you will have a large tax bill because the vest value is treated as ordinary income and taxed at that rate. I do know some people at my company that intentionally set it to the lowest possible % and invest the difference while paying quarterly estimated tax payments, but it’s too much of a hassle for me personally.