Why is the last part(transmit) of the Feynman learning technique optional?

From your source:

If you really want to be sure of your understanding, run it past someone (ideally someone who knows little of the subject). The ultimate test of your knowledge is your capacity to convey it to another. You can read out directly what you’ve written. You can present the material like a lecture. You can ask your friends for a few minutes of their time while you’re buying them dinner. You can volunteer as a guest speaker in your child’s classroom or your parents’ retirement residence. All that really matters is that you attempt to transmit the material to at least one person who isn’t that familiar with it.

This step being "optional" in this particular write up (it isn't noted as such everywhere) is just a practical matter; the author does not want you to be discouraged and feel the technique cannot be applied when you don't have someone else to teach to or don't feel comfortable doing this. It's suggesting that it's ideal to spread what you've learned as a final check on your knowledge, with some ideas here of how to accomplish that, but failing to do so doesn't make the other steps useless, it just makes it slightly incomplete.