An _id when it is an ObjectId is not a number so you cannot $sum. If you want to count using sum you need to use
sum : { $sum : 1 }
Next time you publish code sample please post it using text format rather than a image. This way we can cut-n-paste into our system for experimenting and cut-n-paste into our replies.
I believe this is expected, since the Atlas aggregation pipeline builder is primarily designed for building pipelines, rather than executing them. That is, the Atlas pipeline builder allows you to easily plan & design your aggregation, and once you finished designing them, you should be exporting the finished pipeline to be used in a driver. The pipeline builder provides an easy way to export your pipeline.
In fact, the aggregation pipeline builder you see in Atlas is the same one that you see in MongoDB Compass. However the pipeline builder in Compass has a configurable limit which defaults to 100,000 documents (the same limit as you observed in Atlas). From the description of this setting in Compass:
When Sample Mode is enabled, specifies the number of documents passed to $group, $bucket, and $bucketAuto stages. Lower limits improve pipeline running time, but may miss documents.
But while this limit is configurable in Compass, presently I don’t think it is configurable in Atlas.