Google Treats Hreflang in Sitemaps and HTML the Same
When Google processes a page, it treats hreflang tags in the sitemap and in the page’s HTML the same.
This comes from a short exchange Google’s John Mueller had on Twitter:
sure— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 12, 2018
He also clarified, later in the exchange, that putting hreflang in a sitemap vs. in the page HTML doesn’t affect when Google processes the annotation, assuming Google’s seen the sitemap before.
No, processing would be at the same time as with on-page annotations (assuming we've seen the sitemap beforehand, which is the general case).— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 12, 2018
So, it’s much easier for the website owner to manage their hreflang tags through the sitemap, but for SEO purposes it won’t have a direct impact. Since hreflang implementation can help make or break your international SEO, website owners should stick with the method that works best for them.