dmidecode --type memory showing wrong DDR type?

The dmidecode program gives you the DMI information as reported by the BIOS. It is as accurate as the BIOS makes it. Inaccuracies are common.

You'll notice the BIOS didn't populate the manufacturer or serial number fields, so it's not bothering to provide any more information in the DMI table than it thinks the operating system needs.

To get accurate memory information, you should interrogate the SPD chips on the actual memory sticks.