This is a really questionable statistic. Your link cites a blog from a jobs site. They don't provide any citation for the 500,000 number.

Both sources refer to it as the "number of job openings" that year (2017). Obviously this isn't new demand for software engineers, most job openings are vacancies caused by people moving in the industry and will be filled by other people moving. If there are around 1.5 million SEs in the USA and average tenure in a job is 3 years then this explains all the job openings right there.