The pair were due to appear together at last year’s Dmexco, but speculation around acquisition moves for Twitter from the likes of Google, Salesforce and Disney meant that Dorsey was forced to stay in San Francisco – with the chat carried out over video link instead.
Those approaches fizzled out, and in response to a question from Sorrell, Dorsey confirmed it had been his decision not to enter talks to sell Twitter.
The takeover speculation had been rife, according to Sorrell, because "Twitter does not seem to have achieved the prominence that you would want." He noted that in 2016, WPP had increased its spend on the platform from around $240m (£181m) to $300m, but that this figure would stay the same in 2017.
In contrast, the advertising giant will this year spend about $5.5bn with Google, and $2.2bn with Facebook.
Defending the platform, Dorsey said Twitter had achieved brand awareness that was "unlike any company", with the world’s most influential people using it. "A conversation on Twitter is alive, it’s 24/7 and it’s electric he added.
"Has it always been as focused, as simple as streamlined as it needs to be? No. But we’ve focused on providing daily utility to people, and we’ve shown that that continues to accelerate."
Dorsey added he was focused on using data sdcience and deep learning to make features like the timeline more relevant to users, and said: "Every time we make the timeline more relevant to someone, usage goes up."
Sorrell also suggested that Twitter had been a "clunky" experience for advertisers. Dorsey admitted it had been complicated to use, and also said; "We’ve not focused enough on ROI and proving that it works. We want to make sure every advertiser coming to Twitter can prove that it works."
Dorsey on Trump
Of the world’s most influential people mentioned by Dorsey, the one that has perhaps most defined Twitter in the last two years is US President Donald Trump.
Dorsey chose to dodge a suggestion from Sorrell that Trump’s presence on Twitter was worth about $2bn to the company's valuation, but did discuss the ethics around providing a platform to divisive figures.
"I think it’s really important that we hear directly from our leaders," Dorsey said. "And I think it’s really important that we see how these leaders are thinking so we can hold them to account. If we don’t have those out in the open, then we’re in the dark."
But Dorsey also said that both he and Twitter had been "very vocal" about the policies he disagreed with – not least the recent move by Trump to end protection for immigrants who arrived in the US as children.
Asked by Sorrell if he had been invited to participate in Trump’s business council, Dorsey said: "Not that I’m aware of." If he had been, he would have "had to evaluate" whether it was the right move to accept.
Commenting on the skinny jeans and oversized black hoodie worn on stage by Dorsey, Sorrell recollected that last year, "you were in San Francisco in your pyjamas – some would look at you and say you’re still in your pyjamas."
Sorrell joins Twitter
The pair also took a selfie photograph together, with Sorrell remarking that the picture was for his Twitter account - @martinsorrell – and joked that he has "finally succumbed" to joining the platform. Dorsey left the stage saying "Follow this account".
Sorrell joined Twitter after a deal he and Dorsey struck a year ago: if Dorsey attended Dmexco in person (which he was supposed to do last year), Sorrell would join Twitter.
Although Sorrell tweeted for the first time today, above, the @martinsorrell account was created in February 2009.