Found it. We are both right.
Note that this is the url to the feature layer item, rather than the url of the feature service itslef. In old versions of Survey123 we actually supported working directly with feature service urls, but when you let Survey123 Connect create your initial XLSForm design, the feature layer item url will always be used. Right now, Survey123 supports feature service urls only for backwards compatibility. you should always use a url to the item!
What is weird is there is no easy way to get the URL that uses itemid. How does survey123 expect you to get this URL?
If I go to the Rest page and look for the Service ItemId: then I can find the item id.
Then I have to know to use this start https://www.arcgis.com/sharing/rest/content/items/ and add that item id on at the end.
When you try and look at this URL I just get a blank page with API reference on it - which makes it seem wrong.
Is there a easier way to get this url? When you do to the Item page of the service at the bottom it says URL and gives the /FeatureServer one. So it would make sense that is what you want. Old way seemed a lot easier and less confusing.
For the poster I think it is this Never use the Token that is just to get in for security
https://www.arcgis.com/sharing/rest/content/items/988dddec82b9478ab2a89ac32058adb9