As far as I've seen, the previews of an email in Sales Insight don't have any of the tokens or dynamic content populated.
This is just the preview though, the tokens/dynamic content are rendered in the actual email sent to the lead.
My guess is it's just easier to store one version of the email within SF for preview, rather than store every leads' variations.
After digging into some additional tests we were able to verify that this previewer just seems to be inconsistent.
Like you said, we've encountered plenty of instances where the previewed tokens on the SFDC side never pull in anything other than defaults, but it was the inconsistency of accurate tokens and defaults that had us concerned with how they were being actually delivered.