Why do they think that WDJ0551+4135 is "snowman shaped" i.e. a contact binary?
It depends who the "they" is in your question.

If by "they", you mean the astronomers who discovered it, they don't. The paper (which is available on arXiv) clearly states it formed from a merger, and doesn't mention the term "contact binary" at all. The merger has already happened long ago, what we're seeing is the end result of it.

If by "they" you mean the BBC, it looks like their correspondent has gone and read far too much into the artist's impression attached to the press release. The artwork depicts the final stage of a merging white dwarf, rather than the object as it appears today. It's probably understandable since a contact binary does look quite bizarre compared to the usual spherical star, and the title was "Astronomers find a bizarre, dying star that 'didn't make any sense'".

There's definitely an argument to be had about whether artists' impressions are misleading when attached to this kind of science communication, but that's something for another website...