According to the website, both teams are said to have worked independently until 2015, when Apple wanted to launch a speaker without voice assistants until the Amazon Echo - but the echo changed this.
At the same time, Siri is said to have owned a hard-to-scale infrastructure that caused years of trouble. For example, the publication far exceeded expectations. The large rush of users worsened the functionality in addition, since the system was not designed for the large amounts of data. As a result, the Siri code was drastically changed so that you could lower the required servers for a component from 500 to 5. Even at Steve Jobs' lifetime, Siri was to be released to developers, but the plans were postponed because the code was not ready yet. Therefore, the release took place much later.
While Apple took over the companies Topsy (2013) and VocalIQ (2015) in the following years to improve Siri and to solve the previous problems, the team learned behind the language assistant in 2015 from Apple's HomePod. Together with employees of the Beats division, they are now also working on the speech recognition and the arrangement of the microphones.
Incidentally, Apple's SVP Craig Federighi took over the Siri team last September. Under his leadership, the language assistant is now overcome the last technical hurdles and show his full potential. Maybe already at this year's WWDC a first look into the future can be given.