While I agree some might see their reach go down when using a publishing tool, I would account that as penalized for not being active on the social media site , not related to how you actually post the content.

A lot of brands start posting from a publication tool and then basically never goes in-platform to engage with any content, which is where I’d flag the drop in performance happens.

To your client – I would say that this would be a major factor, and explain how you’ll be staying active in-platform to fight against this.

Source: in the industry for 10 years, 5 of which I ran an agency where I ran into the same issue.

Good luck!