It makes no sense, or very little. These TikTokky non-members bring in zero revenue and generate zero content that would be of interest to potential paying members. In fact, they 'lower the tone of the establishment' and potentially drive paying members and quality writers away.
It is absurd to fawn over the phenomenon. Presumably the hope is that 1% of 10,000,000 TikTokky browsers sign up for a sub, but if that's the marketing strategy, it seems as targeted and intelligent as old-fashioned letterboxing or spam.
Medium constantly seems to defy the expectations of a data-driven, 21st-century, California-based tech company.