[Khoth]

It's mostly talking about the case where someone receives an encrypted message which is intended to later be published openly. If it was padded by adding stuff, an attacker can try to reconstruct the original plaintext by removing the flowery adjectives, whereas if things were deleted the attacker doesn't know what to add.

[manwe150]

In particular, the length of a message is not encrypted when encrypting the text. So if the encrypted message is shorter, you know exactly how much to remove to get back the original, and then just need to guess what to delete. If the message is longer, it is much harder to guess whether to add flowery adjectives, a new sentence, change a pronoun for a name, or some other change.