Webflow always uses the HTML5 tag that you specify for each element.
Since section tags are not necessarily meant for ‘page sections’, we leave it up to you to decide which tag makes more semantic sense. You can change the HTML5 tag type in the Settings pane for any element and it will update in the exported code:
Some side info from HTML5 Doctor - http://html5doctor.com/the-section-element:
The section element represents a generic document or application
section…The section element is not a generic container element. When
an element is needed only for styling purposes or as a convenience for
scripting, authors are encouraged to use the div element instead.
Since in the Slate template the <div class="section"> elements are more like generic containers, we chose to default to <div>s instead, but it’s kind of a grey area - I think we could have gone either way 