We had the same concern. So we created a new site collection specifically for surveys. Then we turned on the setting "Allow both external users who accept sharing invitations and anonymous guest links: in our Tenant, and turned it off for all site collections except the one we will use only for Excel Surveys.
The answer is that the one user that was used to create the Office 365 account needed to have its login updated - it was still set to @company.onmicrosoft.com and updating it to @company.com, logging out and logging in fixed the issue. All future accounts created won't have this issue as they'll be @company.com by default.
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