I think this is mostly a pedantic point for a late stage private convertible preferred.
While private convertible preferreds typically have a dividend stream, conversion option (typically at spot) and liquidation preference (1x and presumably no participation feature in this case), they are theoretically more valuable than common. However, since there is no public market for the common, hedging that option wouldn't be efficient (if feasible at all) so I don't think the valuation really reflects that additional value (if at all). Investors develop a valuation view based on the metrics they would use for common (growth, fundamentals, perhaps some real option value, etc.) and invest based on that. All the preferred features are in the doc to provide some protection in the downside, which I don't think is much of a consideration in case of Stripe. You can see this pricing dynamic in S-1 filings of a few precedents (Twitter comes to mind) where founders were able to tender some of their common in the later rounds at the same valuation as preferred[1].
On the other hand, you are spot on when it comes to a public security (i.e. post-IPO convertible preferred, mostly capital/ratings instrument and quite uncommon in tech) - you would definitely bake-in the option value and dividend stream into the security valuation which would result in some conversion premium for the company and say 5-10 points in theoretical value above the par for investors. The difference is that primary buyers of the public convertible preferred security would be hedge funds who can short the common and effectively monetize the option value embedded in the security.
If anything, the way I would look at this is that any credible bid for Stripe would probably have to be in 4-5x of that valuation.
[1] See pgs. 139 (bottom, 2011 Third-Party Tender Offer) and II-3 (top) of Twitter S-1 (http://1.usa.gov/1cEqy0J) for difference of ~1%, likely due to fees, etc.