Does Windows Server 2019 Essentials have to be a domain controller as requirement?

No, it doesn't have to be configured as a domain controller, but can be so configured.

The Microsoft blog Windows Server 2019 Essentials update has this to say:

If configured as a Domain Controller, Windows Server 2019 Essentials must be the only Domain Controller, must run all Flexible Single Master Operations (FSMO) roles, and cannot have two-way trusts with other Active Directory domains.

In short, configuring it as domain controller will have drastic consequences for the network in which it is found.

I strongly advise to skip Windows Server 2019 Essentials and get instead Windows Server 2016 Essentials.

The article Windows Server 2019 Essentials – end of an era! says this:

While Microsoft will still be releasing an “Essentials” SKU for Windows Server 2019, it will be completely devoid of everything that we recognize as being part of Windows Server Essentials in its current form.

Basically, Microsoft is releasing their first (and last) non-essentials Windows Server Essentials release that is completely gutted of anything even remotely resembling Windows Server Essentials as we currently know it. And obviously, without the underlying “Essentials” bits, there can be no more new WSE RemoteApp or WSE WorkFolders product releases going forward.

My suggestion to those who want to continue running an on-premises Essentials server would be to grab up as many Windows Server 2016 (Standard, Datacenter, or Essentials) licenses as you possibly can (before they’re all gone), and then ride it out with that particular version for as long as Microsoft will be providing support for it (see: Windows Server 2016 Essentials servicing timeline).