No, dividing the number of reported cases by the number of reported deaths absolutely does not give the infection fatality rate. For example, the UK has one of the most widespread testing programs in the world and also monitors the actual infection rate via sampling, and we're still not detecting most Covid infections and reporting them as cases. I think for a while the government was even publishing estimates of what proportion of infections were being found, and it wasn't much, can't find them off-hand though. Covid-related deaths, though, are pretty well reported since pretty much anyone touching the medical system gets tested. (Some countries have been massively underdetecting deaths but they're making an even worse job of reporting mild cases...)