Does that suggest that this ESRI shop could now read their field data into Manifold and ESRI?
There are three levels of compatibility.
The first level are the forms used. Both Survey123 and KoBo use Xform internally, saving and reading that Xform using XLSform format (a simple Excel xls file) for interchange. If you have a cool Survey123 form that you're using and you want to share it with somebody else, when you save it, it saves as an XLSform in .xls or .xlsx format. If you go to Esri's web sites, the various Survey123 samples and forms both Esri and other users have shared online are in that format.
My experience is that over 90% of those forms saved from Survey123 will pop open and can be used unmodified if you upload them into the KoBo form designer. That makes sense, since both Survey123 and KoBoToolbox forms are built using Enketo Core or Enketo Express.
As for going the other way, saving a form created in KoBoToolbox and then reading that .xls into Survey123, I don't have personal experience on how good the compatibility is, but I've heard that Survey123 will read virtually all of them unmodified, especially if it is a plain vanilla form that uses typically-used features.
In either case, it's easy to modify an XLSform: just pop the .xls or xlsx open in Excel or in any of the open source (Libre Office, etc.) spreadsheet programs. The forms are really simple worksheets that are easy to edit however you like.
The second level of compatibility is what server is used to collect the data from the form. KoBoToolbox uses KoBoToolbox servers, either provided free by the KoBo organization or a KoBoToolbox instance running on your own server. Survey123 uses Esri's AGOL (ArcGIS On Line) servers. KoBoToolbox forms running either in KoBoCollect or as a webform will not sync to AGOL servers. I don't know if Survey123 has the ability to sync with a generic ODK or KoBOToolbox server. My guess is that Esri has made sure it requires AGOL.
The third level of compatibility is for a GIS application like ArcGIS Pro or Manifold to be able to get the data that has been synced into servers from the mobile collection effort. With ArcGIS Pro, you're part of the AGOL system, so you can connect to server side storage, with the data staying in the AGOL server, just linked into your ArcGIS Pro project. To get data from there into local storage, you could save it into a local geodatabase.
The new KoBo dataport gives Manifold an analogous capability. Connect to the KoBo server, just like Arc would connect to the AGOL server, and the data appears in a data source in your Manifold project, but still resident up in the KoBo server. To get that data local, it's easier with Manifold since you can just copy and paste it into the project.
As for Esri shops reading their field data into Manifold as well as Esri, that's easy to do. They'd just grab the data the way they always have and save it locally in a geodatabase. They've just now read their field data into an Esri format for local use, with no need to connect to AGOL to get it. Manifold can open that geodatabase and use the data the same as ArcGIS Pro.