The authors acknowledge the issues you cite. Headline writers are not bound by such niceties.
From the survey PDF:
"First, our methods are imprecise in measuring a person’s Bitcoin holdings. Second, and
related, standard errors are quite large, resulting in a large range of feasible values. In
particular, the 95 per cent confidence interval for the share of Canadian Bitcoin in 2017
ranges from 5.7 to 16.8. Finally, using multiple imputation requires the assumption that
missing values are missing “at random.”
Further, there are reasons to suspect that we are specifically overestimating total Bitcoin
holdings in Canada."
https://www.bankofcanada.ca/wp-content/uploads/2018/07/san2018-23.pdf
I have some other concerns also - I could find nothing to indicate whether respondents knew that the survey was sponsored by a central bank.
However, most central banks are actively hostile to crypto and the Bank of Canada is certainly among them. For example,
this is in the introduction to the survey publication itself(!):
"Actually, there is one more thing keeping me awake at night, which perhaps
I should mention, and that is all the noise I keep hearing about cryptocurrencies,
especially Bitcoin. There is a lot of hype around Bitcoin, and markets are evolv-
ing quickly to allow wider access, including to retail investors. So perhaps you
will allow me to make a couple of points. To begin with basics, the term ‘cryp-
tocurrency’ is a misnomer—‘crypto,’ yes, but ‘currency,’ no. For something to be
considered a currency, it must act as a reliable store of value, and you should be
able to spend it easily. These instruments possess neither of these characteristics,
so they do not constitute ‘money.’ ”
Governor Stephen S. Poloz, December 14, 2017"
I am as skeptical of this survey as I would be of a survey by any other special interest voice.