If you read it, it sounds easy. The "easy" things in their categorization are things like removing the bottom cover. If you're not comfortable doing it, I believe you can ship it to them and have them do it. (If you have questions about these things, I highly recommend asking them.)

Sure, it's not as easy as an external battery, but I've not seen those in a long, long time. But you also don't need specialist tools nor a 35kg custom kit to replace it (https://www.theverge.com/2022/5/21/23079058/apple-self-servi...). And you definitely don't _have_ to send it to them to get it changed.

For contrast, here's Apple's page on how to replace their batteries: https://www.apple.com/batteries/service-and-recycling/