Just think in LOGICAL priorities where higher is above lower and lower is below [edit!] higher - because you are turning everything in knots. Ignore the actual numerical values - as I said originally - ARM MCUs use low NUMERICAL values to represent high LOGICAL priorities - but nearly every other MCU (and FreeRTOS runs on 40+ architectures) do it the other way around. Therefore the website cannot make an assumption as to which way around a particular MCU happens to number interrupt priorities - but no matter logically higher interrupt priroities are always higher in priority than logically lower interrupt priorities - that is why they are called priorities. If you mask up to a logical priority level then only interrupts that have a logically higher priority can execute.
Therefore configMAX_SYSCALL_INTERRUPT_PRIORITY is the maximum LOGICAL priority from which a FreeRTOS API function can be called.
When the MCU masks interrupts up to a specified LOGICALLY priority value, all interrupt priorities LOGICALLY lower than it (the ones that the MCU sees as having lower priority, doesn’t matter what its actual number is) are masked and interrupt priorities LOGICALLY higher than it (the ones that can still interrupt because they are not masked) can still execute.