I am not sure if the understanding of SEO is correct here. A SEO analysis you page, the traffic flow, the link structure, rankings, CTRs, SERP features, loading speed, structured data, images, videos, semantics ... just to name a few. For this the SEO needs data from sources like GSC, Analytics, Matomo and other tools.
This will result in recommendations to improve Ranking, Traffic, CTR and conversions on your page. The implementation of the changes in normally up to your inhouse content and dev team.
If you want the SEO (no matter if agency or freelancer) to make the changes, because you don't have the resources for that, than it is something different. They will need their own content writer and have to understand how to implement the changes (But this should be no biggie in any CMS). If your page is hard coded using a Klingon version of C++ , than I wouldn't be too surprised that they tell you that they can not be of any help.