Digital platforms are often theorized as universal drivers of political polarization, yet empirical patterns remain inconsistent across countries. Leveraging the natural-experiment logic of Sweden's and Finland's simultaneous 2022 NATO applications and the NATO–Turkey diplomatic crisis, this study examines how national political cultures condition the enactment of identical Twitter/X affordances. Using a most-similar systems design, it analyzes geolocated posts from the Nordic Tweet Stream (January 2022–May 2023). Communicative structure is measured through participation inequality (Gini; top-share-of-voice), and communicative behavior through @mention and #hashtag densities, complemented by qualitative profiling and a supplementary stance and framing analysis. Sweden exhibits a more concentrated, broadcast-leaning configuration (Gini = 0.59; top 1% produce 34.1%; lower mention/hashtag densities), whereas Finland shows a more distributed structure with higher interaction/coordination proxies (Gini = 0.46; top 1% produce 20.4%; higher densities). A stance analysis reveals that both publics support NATO accession, with anti-accession sentiment absent; the divergence lies not in substantive political position but in communicative repertoire, with Swedish discourse dominated by adversarial-blame framing and Finnish discourse by analytical-informational framing (χ2 = 117.40, p < 0.001). The findings support a “cultural filter” operating as a framing selector, linking political culture to divergent configurations of platformed publics.