Why can't I update my Ubuntu from 18.04 to 20.04?

The -d flag bumps you to the development release allowing you to bump before a release has been officially opened for upgrade.

The upgrade from 18.04 to 20.04 is now officially opened so -d doesn't apply to that release-upgrade (it did straight after 20.04's release hence the blog notice, until it was declared stable thus wasn't required from then on).

Upgrades to the development release are only available from the latest supported release.

Now the -d means bumping from 21.04 (ie. latest release) to the development or impish release (what will be 21.10 on release), but you're not on 21.04 - thus the error message you received.

--- Additional detail

Why is -d necessary for a LTS release?

The upgrade taps are documented as being turned after the release of 20.04.1 and the Ubuntu Release Team considers it fully stable for existing users. Many people however don't want to wait for this, so the -d allows them to upgrade anytime after 20.04 was released, even before 20.04.1's release.. or in the days between 20.04.1 being released (date of release being the ISO release for new installs) and the taps being turned on for upgrades (usually 2-5 days later).