Why is there a 100m external patch lead if an Ethernet channel is normally limited to 100m?

Because cable assemblies are used in more than one kind of installation, but the existence of a 100m cord does not rewrite structured-cabling rules.

As an Ethernet channel, a 100m patch cord by itself reaches the familiar nominal channel distance before any extra patching is added, and its stranded construction can require additional allowance.

For a standards-critical 100m copper installation, design and test the complete channel rather than treating the number on the packet as a guarantee.