Looking at the manual, https://matrix.squiz.net/manuals/keyword-replacements/chapters/custom-form, I don’t see a keyword replacement for section summary. If you only want to see the responses for some sections you might need to manually add each question using %response_<parent_id>_<question_id>% and group them under each section title using %section_title_<parent_id>%
Hmm I didn’t realise there was that page, seems a little different to the page I linked to.
Anyway, I tried out %section_summary_{section_asset_id}% on our system (Matrix v5.5.2.1) and it worked fine. I saw the section summary on the thank you page and in the submission email.
Is “708252” definitely the ID of your Form Section asset? Make sure you are not using the ID of your Form Contents asset. Maybe you could post a screenshot of your asset tree?