When I look at the raw feed I see they have their dates in <dc:date> elements which are not part of the RSS specification, but an extension to RSS that uses Dublin Core. The Argotic Framework we use to parse the feeds is ignorant of that extension and expects only the standard <pubDate> element which is not included in the List Apart feed. So mojoPortal needs a date there and uses the current date since it has no access to the date in that <dc:date> element.
To me it is a bug in the feed if it does not have the standard elements. This blog post talks about those 2 different date formats and the author recommends that RSS feeds have the <pubDate> field to avoid problems with RSS readers that may not recognize the Dublin Core elements. They could have those Dublin Core things in addition to the standard RSS fields but leaving out the standard fields is a bug in the feed. Even including Dublin Core elements it seems like there should be an xml namespace declaration for those elements at the top of the feed but they only list RSS and atom. So any way I look at it the feed is missing expected elements and including undeclared elements.
I don't have any immediate solution. You would think a very reputable site like A List Apart would have a good feed, looks like it runs on Expression Engine CMS so really its an issue/bug in the feed generator in that CMS.