I'm still learning Cybersec in general, if I'm reading this right, were these credentials hardcoded in by D-Link, these devices reached EOL, and so they refuse to remove that backdoor on the basis that the devices are EOL?

Is there a likely reason that these were left in, like could it have been a development oversight, or does it look more likely that this was malicious?

Regardless, I definitely hold the opinion that D-Link should do the right thing for their customers and patch that vulnerability, regardless of the device being EOL, similar to how Microsoft pushed a security update to Windows XP re WannaCry when it was EOL, on the basis that "Yes, XP is unsupported and you shouldn't use it, but we are patching this particular vulnerability anyway."