The employees' shares are tax based on the IPO price, not the price at which shares beginning trading.
So with a large bump (like 100% on Snowflake for example), you defer a lot of tax to the future. And if you hold out 1 year until selling, you change a lot of that gain from income into long term capital gains (which has a lower tax rate)
Any actual shares an employee owns at the IPO will not be taxed until they sell them. You might be referring to the synthetic RSUs that companies have been issuing that convert to real RSUs at the IPO (or acquisition) which does incur taxes.
So with a large bump (like 100% on Snowflake for example), you defer a lot of tax to the future. And if you hold out 1 year until selling, you change a lot of that gain from income into long term capital gains (which has a lower tax rate)