Just got a reply from them, confirming my thoughts, please see below.
----
Thanks for reaching out to us.
Indeed, if you make a topup into your Borderless account (be it personal or business), the money needs to come from︀ a bank account that belongs to the account holder.
So, if you make a topup︁ to your business account, the money needs to come from a bank account registered in︂ your business' name. If the topup is going on your personal profile, the money has︃ to come from your personal bank account.
There is unfortunately no way around this. If︄ you don't have a bank account for your business, then we sadly can't accept a︅ payment made into your business account, funded from your personal account. We hope you understand.︆
If you have any further questions, don't hesitate to contact us again.
All the best,︇
Elis