The eventual answer for this was to ignore the docs and specify both an app section and a DOMAIN_INSTALLABLE and GOOGLE_DRIVE container. Here's our manifest, which is working great - it can be installed from the "Add Marketplace Apps" button in admin.google.com, and also installed from the Chrome Web Store.
{
"manifest_version": 2,
"name": "Pear Deck Beta Preview",
"version": "0.0.0.10",
"description": "for Active Classrooms",
"icons": {
"128": "YOUR_ICON_128x128.png",
"16": "YOUR_ICON_16x16.png"
},
"container": ["DOMAIN_INSTALLABLE", "GOOGLE_DRIVE"],
"api_console_project_id": "YOUR_ID",
"app" : {
"launch" : {
"web_url" : "https://peardeck.com/home"
}
}
}