The way I interpret the ZIMRA ruling is that the invoice must be issued from a Fiscal Device.
This link https://www.zimra.co.zw/domestic-taxes/corporate/fiscalisation-explained goes on to explain it in more detail, but it appears a computer does not qualify as a Fiscal Device.
There are three broad categories of the prescribed fiscalised devices and these are;
Fiscalised electronic registers, also referred to as electronic tax registers (ETRs);
Fiscalised printers; and
Electronic signature devices (ESDs)