There is no other option yet.

Did you received any error while downloading feed with starred items?

You could mail support@bazqux.com and send URL that didn't worked so I could check it.

The solution below by appending to the URL didn't work for me. Is there any other way to export all starred posts

Most probably reading app shows starred items from the last month/year (should be somewhere in app settings). So if you star less items this month/year total count will decrease.

Starred and tagged items are kept forever and are not removed. You could check total number of starred items by searching for "*" (everything) in "Starred items" section and look for "Found N items" text.

As a workaround till export will be implemented you could export last 1000 starred items as RSS if you make public feed and add ?n=1000 to the URL (it actually possible to export all items if you look for <gr:continuation>NNN</gr:continuation> parameter in feed and add &c=NNN to URL, to get next 1000 items and so on).

I noticed the total number of starred posts I have is decreasing in my reading app. As far as the web interface goes, there's no way to tell how many starred items I have. This seems kind of important to me, to be able to export my most important historical data. I'd also like to know if the oldest starred posts are gone or what.

You could use the RSS Feed as an IFTTT Trigger, so this would be useful for all the people voting for an IFTTT channel ...
https://bazqux.uservoice.com/forums/190752-general/suggestions/4127243-ifttt-channel

This would be great. Used to have my starred google reader articles backed up to Pinboard. Would love to be able to do something similar.