I don’t understand why as yet, but from an internet source, found this amendment to the composer.json file allows the mongodb/mongodb package to install, the new "provide" section:

{
    "require": {
        "mongodb/mongodb": "^1.8.0",
        "vlucas/phpdotenv": "^5.3.0",
        "auth0/auth0-php": "^7.9.0"
    },
    "provide" : {
    	"ext-mongodb": "*"
	}
}