This is a bit of a misleading summary.
Melenchon speaks for his own party, France Unbowed (LFI), not the entire NFP alliance.
The NFP as a whole has not declared support for Melenchon's position, although his party controls 71 (~41%) of NFP's 180 seats in the National Assembly.
Macron has already indicated that he will not allow Melenchon to become Prime Minister, and the entire NFP will be aware that they must select a more moderate leader to represent them if they expect to gain enough support from the centre to operate as a minority government.
Your instinct to search for a non-Murdoch source is correct, as they are trying to paint the entire NFP as holding the same positions as their most extreme member.
There is little to no chance of Melenchon becoming Prime Minister or having any ability to enact this tax.
However a marginal tax rate of 90% on income over €400k is well above the upper-middle class and would apply to only the wealthiest families, most of whom would still have other avenues to minimise the tax they actually pay.