Tried to follow Boglehead advice on HSA investments and ran into a snag

This is the email I just received from HR;

Following the transfer request you made with Optum, your account was automatically closed. We have re-enrolled you in their portal. If you haven’t done so already, please register after three business days from today to ensure you still have access to your account.

Please be advised that you are not allowed to make any transfers out of your HSA account while actively employed, as doing so will cause your account to be closed again. Transfer requests are only permitted for terminated employees and not for active employees.

If you feel that Optum Bank is not a vendor you wish to work with, you can stop your HSA contributions at any time. Enrollment in this benefit is voluntary. However, if you decide to continue contributing, please note that transferring funds out (even if it’s not the full amount) will result in your account being closed and it will not be reopened a second time.

I hadn't realized when I made the transfer request from Optum to Fidelity that a full transfer would close the account, I now know and understand that. But secondarily, are employers allowed to enforce this kind of restriction? I thought that employers had no control of funds once they deposited them into an HSA. Is this just someone in HR that has no idea what they're talking about? Or is it possible that my employer actually has this policy? Should I ignore her and just transfer a partial amount next time so that it doesn't get closed since there's little to no way they could know about it regardless?

Edit: Just got a response from HR after telling them I was pretty sure they were wrong and yep, higher level HR said that previous rep was just wrong, no issue now.