A recent paper on binding constraints to Malawi's growth :
The authors find that growth in Malawi has been primarily driven by the domestic multiplier effect from export revenues. The multiplier effect is particularly pronounced due to the high number of smallholder farmers, which produce Malawi’s main export crop, tobacco, and consequently results in the widespread and rapid transmission of agricultural export income. Furthermore, despite changes in the structure of agricultural production from estate to smallholder farming and liberalization of prices and finance, a longstanding relationship persists between exports in real domestic currency and overall gross domestic product.I suspect the above conclusion and this outburst from Mutharika are linked. The constant drying up of foreign exchange certainly appears to point to an overvalued exchange rate. That said in general I was not confident the authors have fully demonstrated this is the "most binding constraint". Its one for further study, especially given the other constraints identified e.g. high cost of finance.
This central role of exports in creating domestic demand highlights the importance of the real exchange rate in Malawi’s growth story, which directly increases the strength of the export multiplier. The most pressing constraint to growth in Malawi continues to be the regime of exchange rate management. Despite good progress, there is compelling evidence that the rate is still substantially overvalued. Furthermore, it is also likely that the inflow of foreign aid—in excess of 50 percent of exports contributes to the overvaluation through its large component of recurrent expenditures.