WP engine never modified WordPress. They took stock Wordpress and edited a configuration file to disable revisions. They didn’t actually change any code.
I thought they'd also modified revisions to have a limit, instead of unlimited. Even when 'enabled', it's not how the rest of the wp installs would behave (iirc). Likely there was some code change there to enable that restriction.
No. They add the below to wp-config.php:
define ('WP_POST_REVISIONS', 3);
They use the parts of wordpress that are specifically built in to make modifications, as any other site maintainer would hosting their own install.
You are mistaken.
You're out here on HN acting completely unfamiliar with the idea of configuring to limit resource consumption?