Often times secrets are numerical constants. In your own example, the icon is a base64-encoded number. How would you tell secret numbers apart from the rest?
Base64 isn't numeric it's alphanumeric. The only reason this is reasonable (again) is that alomost all secrets like api keys or complex passwords are maximizing their information content and are therefore alphanumeric (or better). Base64 encoded data does too, and is an innocent casualty in that censorship.
They meant that a number written in hex (base16) is still a number, even though you use some letters. Similarly, a number written in base64 is still a number.