You're right, I know nothing about ISPs in Poland, but if what you're saying is true, the "downright irresponsible" part belongs solely with the ISP for not knowing how to architect and manage their network, not with the customer.
For the record, I never advocated connecting any kind of device to an ISP without disabling its WAN-facing DHCP server. I only said doing so wouldn't, or SHOULDN'T, cause any issues for the ISP. Your statement that I somehow gave advice to the contrary is false.