I took a closer look at the page you referred to. Here is what I think the relevant bits:
If you don't plan to use SQL Server Express, set up a supported version of SQL Server. When you set up SQL Server for Azure DevOps on-premises, at a minimum install the Database Engine and Full-Text and Semantic Extractions for Search features. We recommend that you use SQL Server Express only for personal or evaluation deployments. Paid copies of Azure DevOps on-premises include a SQL Server Standard license.
That is, it appears that you are supposed to install SQL Server yourself. It does not come with Azure DevOps.
This means that if you want to use Standard Edition, you need to get hold of the installation media. Standard Edition is not available for public download. Only free versions and evaluation versions are, and it seems that you found the latter. However, Standard Edition is available for download from Visual Studio Downloads. It should also be downloadable from Partner/Volume licensing sites. Basically, the same places that are listed further below on the page you linked to. And shops that have heaps of SQL Server instances has the install media on the shelf.
The last sentence in the quote tells me that if you have bought a license of Azure DevOps, you have also bought a license of SQL Server Standard, but that does not mean that Azure DevOps come with SQL Server.