That is why it is transgender and not transexual. Even then, if you consider sex as "binary", sex is stored as multiple bits encoded as X/Y chromosomes as opposed to 0/1 states and atypical chromosome storage leads to intersex and other anomalies.
The survey is of academics across all disciplines so there is no particularly good reason to treat their views on the topic with particular importance.
There's also this:
58 per cent of respondents think sex is binary, except in rare cases such as intersex individuals
So they're only saying it's mostly binary.
(Censuswide, who ran the survey, as are members of the British Polling Council so we can at least expect them to have run the survey reasonably.)