The post was a bit confusing but I think it’s referring to a specific scenario when starting from the latest Fairphone OS (i.e. the stock one if you bought from FP directly) and trying to move to iodéOS. That transition would result in a hard brick under certain bootloader lock configurations.
If you already have iodéOS 7.1 installed, I believe updating to 7.3 should be fine even with a (re)locked bootloader, but I’m also holding off from updating for now. Another post mentioned problems due to which they skipped 7.2 for FP6, but given that they pushed out an OTA for 7.3 I would assume it’s fixed now?
I’ll probably bite the bullet and try updating soon, but I’d feel safer if there was a confirmation on whether updating 7.1→7.3 is intended to be working now with a locked bootloader 