That link doesn't seem to provide guidance on which format to use.  I don't believe they have a "standard".

In the following article under "Choosing the correct format" Google recommends txt over xml for most small merchants (I think they mean merchants without technical staff, not catalog size).

https://support.google.com/merchants/answer/188478?hl=en

In the following article they go over both options and list pros and cons:

https://support.google.com/merchants/answer/160567?hl=en

Text makes sense for ProductCart because it is human readable, can be opened in a spreadsheet and easily modified without technical coding knowledge.  The advantages of XML do not seem worth losing the ability to easily "tweak" the feed in Excel. 

Did you run into a specific reason why the txt method didn’t meet your needs, or were you looking for XML because you read Google preferred it?

You’ll need to use a feed generator or plugin that supports Google XML format output, not just TSV or TXT.
For example, on cheaplyapps.com, you could:

1. Use your e-commerce platform’s XML feed option – Many platforms (like Shopify, WooCommerce, etc.) have apps or built-in settings to export in Google XML format. In Shopify, you can install an app such as “Google Shopping Feed” or “Data Feed Watch” and set the output format to XML.

2. Convert your existing text feed to XML – If you already have the product data in TXT/TSV, you can run it through a converter or script (e.g., using an online CSV-to-XML tool or a custom script that maps your fields to Google’s XML schema).

3. Manually build the XML using Google’s product feed specification – This is more technical, but it gives you full control over the feed structure and ensures compatibility with platforms that accept Google XML.

The key is that the feed generator must explicitly support Google XML syntax — otherwise you’ll always get a flat text file instead of the structured XML you need.