The explanation is right there in the article, and it does not involve your supposed modern day hubris:

Restorers found that the central panel of the artwork, known as the Adoration of the Mystic Lamb, had been painted over in the 16th Century.

Another artist had altered the Lamb of God, a symbol for Jesus depicted at the centre of the panel.

Now conservationists have stripped away the overpaint, revealing the lamb's "intense gaze" and "large frontal eyes".