Seems like this would be beneficial for Twitter since it exempts them from the extra regulation that being a DMA "gatekeeper" entails. It seems like the main target of the DMA is front-page of the web/device interaction type entities like search results pages or app stores. Being designated seems to mean you're subject to more requirements around fairness, self-promotion, and access rights.

All this gleaned from this official summary article from the EU: https://digital-markets-act.ec.europa.eu/about-dma_en