Confused about how external sharing (extranet) works in SharePoint?

I've been doing some testing around enabling an extranet in our SharePoint environment, a place where we can share certain documents with people external to our business.

I followed one of Microsoft's support guides by doing the following:

In the Admin Center I moved the SharePoint sharing setting up to 'New and existing guests'. I also tweaked the sharing settings on the individual 'Extranet' site collection I'd created to match this.

It seems that I can go to the site and successfully share it with any email address I type in. I tried testing it by using my personal email address, which we'll just say is toby@gmail.com.

I received an email invitation to my personal inbox, so it appears to be working. However, when accepting the link from this email I get the following message. This is where I'm confused.

I'm sure it's a simple solution but I'm finding this message confusing. It's telling me I need to accept the invitation using the account it was shared with, which was toby@gmail.com, but this is where I accepted the invite in the form of the email? It also asks to accept the invitation again with my personal email, but how do I do this? Do I have to create a new Microsoft account?

Any help would be great, thanks.