dealerdirect/phpcodesniffer-composer-installer is a commonly used composer plugin to make it easy to import phpcs standards via composer.

For that plugin to be able to recognize mediawiki/mediawiki-codesniffer as being a package containing phpcs standards, composer.json needs to declare the type as being "phpcodesniffer-standard".

The change needed is very simple:

diff --git a/composer.json b/composer.json
index c24cc68..66cc3bf 100644
--- a/composer.json
+++ b/composer.json
@@ -7,6 +7,7 @@
        ],
        "homepage": "https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP",
        "license": "GPL-2.0-or-later",
+       "type": "phpcodesniffer-standard",
        "require": {
                "php": ">=7.2.0",
                "ext-mbstring": "*",

I'd submit that to Gerrit myself, but my Gerrit account seems to be non-functional.