The blog article links Stefan Lorant's own recollection of the event, but the link is broken (fair enough, the blog entry is from 2010). Fortunately though, the link is archived on the Wayback Machine [0], where we can see it is an article from American Heritage, June 1955.

In the linked article Lorent does not specify when exactly he interviewed Edith Carrow Roosevelt, but I think it is fair to assume that the reference to "in the 1950s" is an assumption made by the author of the blog based on when the article was published, and does not cast any doubt on the timeline.

[0] https://web.archive.org/web/20060507100625/http://www.americ...