For example, horizontal partitioning is often used within the same schema on the same instance in order to have separate indexes for (say) current vs. historical data. That's not what is meant by sharding, however; shards are separate instances.
I think people just use the terms slightly differently. Even the Wikipedia article you linked contradicts itself.
> Sharding goes beyond this. It partitions the problematic table(s) in the same way, but it does this across potentially multiple instances of the schema.
"Beyond this" but still "potentially across multiple instances". Anyway, with your framing sharding kind of becomes "distributed horizontal partitioning".
That's not correct: https://en.wikipedia.org/wiki/Shard_(database_architecture)#... al_partitioning
For example, horizontal partitioning is often used within the same schema on the same instance in order to have separate indexes for (say) current vs. historical data. That's not what is meant by sharding, however; shards are separate instances.