The outside display will still be usable, it will just be a similar aspect ratio to the original iPhone through the 4S instead of everything that has come since.
Really wish people would stop comparing it to the mini though, it’s not comparable.
Diagonal corner to corner measurement is not the same as actual screen area or volume, where the folding iPhone will still very likely dwarf the iPhone mini in all measurements.
Not to mention the fact that the folding iPhone will likely have 1.5 MM or so and about 75 G on the Mini.
Anyone who thinks they are going to be able to go from a mini straight to this with very little fuss are not really thinking it through. This thing is going to likely be a very strangely shaped chonker next to the mini.
5.3 vs 5.4 is kind of a pointless measurement, the 8+ was 5.5 and even with bezels gone it’s still significantly bigger than the Mini.