If you only broadcast messages that contain a single block, anyone who already has that block gets no value out of the message. On the other hand, if you broadcast messages generated via fountain codes, messages give you almost a full block worth of information on average.
If you assume the same network environment that the article does, namely one where no feedback occurs from the recipient to the sender, then if you miss a single block in the message, you'll have to wait for the sender to send that one block again. Since the sender has no information about what block you missed, you'll have to wait on average for half the blocks to get retransmitted, resulting in 50% overhead or more. It gets worse if you miss enough blocks that you have a non-trivial chance of missing those blocks again. (On top of that, some transmission mechanisms have properties that make particular data patterns particularly prone to loss, which makes it significantly more likely that you'll lose particular blocks repeatedly.)
With fountain codes and similar mechanisms, on the other hand, every message provides several chances to obtain the missing blocks, resulting in much less overhead.
If you assume the same network environment that the article does, namely one where no feedback occurs from the recipient to the sender, then if you miss a single block in the message, you'll have to wait for the sender to send that one block again. Since the sender has no information about what block you missed, you'll have to wait on average for half the blocks to get retransmitted, resulting in 50% overhead or more. It gets worse if you miss enough blocks that you have a non-trivial chance of missing those blocks again. (On top of that, some transmission mechanisms have properties that make particular data patterns particularly prone to loss, which makes it significantly more likely that you'll lose particular blocks repeatedly.)
With fountain codes and similar mechanisms, on the other hand, every message provides several chances to obtain the missing blocks, resulting in much less overhead.