There are a few things to note here.

For both, around half of GDP is in the services sector. It doesn't matter that much what raw resources they export when it's a smaller portion of GDP (23% in Rwanda, 15% in Malawi). Also, almost all of those raw resources that Rwanda exports are stolen from the DRC by Rwanda-backed militias.

Foreign aid, and Rwanda's ability to position itself geopolitically as a trusted stable partner (which enables more aid and for it to get away with theft and murder) have more impact than maize vs gold.

And in the end, there are plenty of countries that have successfully developed and become less or not poor without having anything of serious value to export. From Bangladesh to the Balkans (I mean post-decolonisation Balkans, not post-Iron Curtain - it was mostly an area with subsistence farming as the main employment, disease, low rates of literacy and very low for higher education, frequent conflict and ethnic/religious tensions, few natural resources other than grain and some very limited amounts coal/minerals). If anything, it usually is the opposite, cf. the resource curse.