That's correct, unfortunately billing testing can only be enabled for the root account user at this time. It is a known feature enhancement to the way role-based permissions are set up in the Developer Dashboard today, but is not prioritized at this time. I'll log this as a +1 on the request internally.
For the time being, the solution you presented below is the best workaround. Or, to preserve the security of your client's credentials, you could have them add your device for you. To do that, they can log into their Roku account, click on My Account, click "+Link a device" under the "My linked devices" section, and then enter the Link Code, which you would have to share with them real-time either over the phone, SMS, Slack, etc.
If you take the approach of having your client link your device for you, then do be sure to have them go into their Developer Dashboard and mark the channel in question as being the Billing Testing channel. If they're not a developer, you may want to walk them through this via a screenshare. The "Enabling billing testing" doc you linked to below includes screenshots of the UI and steps walking through the process: https://developer.roku.com/docs/developer-program/roku-pay/testing/billing-testing.md
