Amazon is limiting it to 5 operations per second per shard, not per customer. That is if you have 200 shards and 1000 write capacity.
It's a problem with how Amazon calculates your capacity units as it scales up Dynamo for you, if you have a particularly large data set with a few very active users.
The parent post is suggesting that 5 Million customers does not in any way imply 200 shards. It's much more likely that with 5M customers you have 1 or 2 shards. That's 4KB (one document) per customer.
It's a problem with how Amazon calculates your capacity units as it scales up Dynamo for you, if you have a particularly large data set with a few very active users.