Just creating a mobile optimized site, is that it?

[frank]

I agree with them if you are running big site with a expectation of big business outcome then you need to do more.

For example, you will certainly a different url for just mobile devices. Also using a mobile responsive theme isn't enough you have to do properly the on-page SEO's such as featuring properly the structured data, hreflang tags, like desktop versions mobile also need a good internal linking structure along with a good navigation system so that. And not to mention but your page should serve dynamic serving too.