Yes, it is indeed true! Anyone with a DWI/DUI conviction is considered to be "inadmissible" and may denied entrance to Canada. In fact, anyone with a criminal record may be denied entrance into the country.
If at least five years have passed since the original conviction date, a person may become eligible to enter the county by applying for (and receiving approval) for 'individual criminal rehabilitation.’ This option involves an application, a fee and other requirements.
In addition, if at least ten years have passed since the completed conviction sentence date, a person may be deemed rehabilitated and permitted to enter the country.