Bad language.
The Supreme Court didn’t “back Trump’s effort”. The Supreme Court took no position (nor should they) on how many workers the Federal government needs to employ.
What the Supreme Court did do was to say Article II section 1 remains in force, that the executive Power of the US is vested in a single individual, at the moment DJT, and that determining the workforce needs of the Executive Branch is included in the executive Power of the US.