That's not true either.
Proposed changes (not accepted by the protesters) are to consider 43% of income as non-taxable personal deductions (up from 20% in the existing law, though actual, receipt-backed deductions for performance of work are accepted), and then pay the regular tax percentages that employees pay, except that Serbian law splits the employer and employee obligations for employees, but charges the sum total of those tax rates in percentage points to freelancers (eg. see the numbers on https://taxsummaries.pwc.com/serbia/individual/other-taxes — so freelancers would pay 25.5% of the taxable income into the pension fund).
A total cost of a single employee for an employer is considered a gross contractual salary (out of which employee taxes are paid from the link above) + employer taxes on top. So because of the slightly lower "base" calculations are done on, actual percentages are slightly lower for employees, but it was basically negated with the 20% deduction. With a 43% deduction, freelancers get to pay fewer taxes than state gets for a single employee for the same net salary.
Eg. if a freelancer ends up with 100k RSD after all taxes, state gets 36k RSD in total taxes: 36k out of 136k gross is not "stupendous" at all! An employee receiving a net 100k RSD salary, state gets around 66k RSD in taxes. If existing law was upheld, it would be around 60k RSD for freelancers (iow, much closer to what employees are paid for).