NAL. The act of following someone with the express intent of collecting genetic material and then covertly sending it off to be processed, without any overall consent, might be considered legal in your jurisdiction. However, you will require a rigorously disciplined approach, in order to acquire a non-contaminated sample, not to mention the costs associated with this exercise ─ just in case, you had any designs of putting this idea on a pitch deck.

Nevertheless, the above-mentioned debate is moot, especially when you consider the original point(s) made in the article, with regards to the vagaries of explicitly sharing your genetic data without first analysing the implications.