Working conditions is things like working hours, your physical environment, your responsibilities [1]. The SpaceX letter was basically "Musk is uncouth, and we don't like that". A fair criticism, but nothing about working conditions.
The text of the letter can be found at [2], if anyone wants to judge for themselves.
[1] https://definitions.uslegal.com/w/working-condition/
[2] https://www.theverge.com/2022/6/16/23170228/spacex-elon-musk...