Those within SEO have long suspected, or known, that UX is a part of the algorithm, albeit indirectly. Google’s former Chief of Search Quality, Udi Manber, testified for the FTC in 2012 with the following statement:
The ranking itself is affected by the click data. If we discover that, for a particular query, hypothetically, 80% of people click on result #2 and 10% click on result #1, after awhile we figure out, well, probably result #2 is the one people want. So we’ll switch it.
Unfortunately, blackhatters would thrive if Google depended too greatly on or was fully transparent about any single factor. Gary Illyes, Google Webmaster Trends Analyst, answered as follows for a Reddit AMA question focused on UX:
PR answer: we use over 200 signals to rank pages. Gary answer: we use over 200 signals to rank pages, and some of those are even announced and you mentioned them.
We don’t need to know exactly why something works to benefit from it. Whether directly or indirectly, great UX improves SEO since it leads to user engagement, with users not bouncing back to the search results. Have a sharp, user-friendly website with a low bounce rate for more organic traffic and conversions.