In the story, we make note of Tropical Storm Arthur's pre-season debut. The storm developed on May 31, one day prior to the official start of the season. We also cite the last time this occured, which was in 1981 with Tropical Storm Arlene.
The aforementioned is not considering subtropical storms, which are non-tropical lows with some tropical characteristics. Most subtropical storms do in-fact start out as non-tropical low pressure systems (what we are used to) and history has shown that they have a higher chance to develop outside the 'official' tropical season.
Subtropical Storm Andrea developed in May 2007 and Subtropical Storm Ana developed in April 2003. Ana is the only subtropical system to develop in the Atlantic in April.
An area of low-pressure also developed over Florida on May 31, 1997. This system was treated an an non-tropical low at the time, but has since been classified as an unnamed sub-tropical low.
So while Tropical Storm Arthur was the first tropical storm to develop before June in 27 years, we have had a couple of subtropical storms develop prior to the start of the season.