score:2
It is difficult to say, but the problem is that the exact definition of legal terms may differ between jurisdictions. There was a similar question here just a few days ago, where a UK visa was denied because the applicant had stated in his application that he was not convicted for a crime and the UK immigration authorities later had a different view of the facts after they had checked the applicant's criminal record. The applicant did have a criminal record, but in his home jurisdiction, the court verdict was not a 'conviction' as of the local legal definition of the term:
Since deception or failure to disclose relevant information in a visa application is usually harshly penalised (e.g. with a long entry ban), if in doubt, you should be open about the issue and give as much details as possible. In this specific case, it is very unlikely that the Canadian immigration authorities would deny you a transit visa even if you answered 'yes' to a question like 'have you ever been deported?' and explained in detail, that you were denied entry because you were not aware that a regular visa was required for transit.