Why do they think that WDJ0551+4135 is "snowman shaped" i.e. a contact binary? The BBC's Huge 'space snowman' is two merging stars says

Researchers have discovered a huge snowman-shaped star with an atmospheric composition never seen before.
It is more massive than our Sun but only two-thirds the Earth's diameter.
The object is thought to have resulted from the merger of two so-called white dwarf stars that often explode as powerful supernovas.
Dr Mark Hollands, of Warwick University, said the team's discovery could help scientists better understand how this process occurs.

and later

But this star, named WDJ0551+4135, has an atmosphere unusually rich in carbon. Dr Hollands said that initially these observations &quot;didn't make any sense. The only way you can explain it is if it was formed through a merger of two white dwarfs&quot;.

I assume this means that it is suspected of being a contact binary
Question: Is WDJ0551+4135 thought to be a contact binary only based on chemistry and evolution arguments? Wouldn't a light curve of this object alone be enough, and wouldn't the period be extremely short making this somewhat easier to do?
update: C|NET's Astronomers find a bizarre, dying star that 'didn't make any sense' suggests that the argument is indeed based on chemistry, and that asteroseismology may be required to back it up. It also links to Nature Astronomy An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant.
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...