There are many people who swear that WordPress is the best content management system (CMS) when it comes to search engine optimization (SEO).
I disagree. When it comes to SEO, the difference doesn't come from the CMS platform you choose. The difference comes from how you use the tools available.
There are many elements that factor into how a site shows up in the search engine result pages (SERPs). Some of those factors are within your control because they're things you can modify within your website. Things like having good page titles, solid content, good internal linking strategies etc. Those are under your control, so in theory, your CMS could make a difference.
Does WordPress do those things better than any other CMS? No. In fact, WordPress in its default setup (as of today's writing) doesn't even allow you to specifically define a page title and meta description for SEO purposes.
What WordPress offers is the potential to simplify your SEO efforts. Add a plugin like WordPress SEO by Yoast or the All In One SEO Pack and you'll suddenly have more options available to you. These aren't native to WordPress. They have to be added in. If you assumed that WordPress was simply better for SEO right out of the box, you'd be disappointed in your results.
A SEO specialist can do SEO on any website. It doesn't matter if it's a static HTML website, a WordPress site, or a site done on any other CMS. I have a static HTML website that's been in the top 10 for a popular search term for over a decade. I have clients with sites built on Adobe's Business Catalyst CMS who have good results in the SERPs.
No CMS offers an inherent advantage over another when it comes to SEO. The difference comes from the human being who manages the site. Anyone who tells you otherwise isn't telling you the whole truth.