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They beat Ireland by 85 runs at Lord's to complete a 2-0 win - their seventh one-day series victory in two years.
"England have got such a strong-looking squad, especially with the bat," Swann told Test Match Special
"It's not long ago they were being thrashed by everyone and insisting they were playing the right way with their 1970s brand of one-day cricket."
Swann, who took 104 wickets in 79 one-day internationals, was referring to the 2015 World Cup when Eoin Morgan's team were humbled by a group-stage exit, in which they only won games against minnows Scotland and Afghanistan.
Since then, England have won series against World Cup runners-up New Zealand, Pakistan (twice), Sri Lanka, Bangladesh, West Indies and now Ireland. They only lost to Australia and South Africa by the odd game in five.
They hammered Ireland despite the absence of key men Ben Stokes and Jos Buttler, who are playing in the Indian Premier League.
Swann added: "They have such a strong middle order. Especially when you consider they can bring in Jos Buttler - one of the best in the world - and add him to that middle order and then Ben Stokes, who is arguably the best player in the world in all formats.
"Eoin Morgan and (head coach) Trevor Bayliss ripped up that piece of paper from 2015 and said 'that's nonsense', we'll get the right personnel in, fill them with confidence, back them to the hilt and ask them to try and post 400 when they bat.
"They scored 328 against Ireland and the captain said he felt they were 40 runs short. That's amazing to hear. Not too long ago, England captains and teams of old would have been cock-a-hoop with a score of 328."
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England are "justified favourites" for the Champions Trophy on home soil next month, says ex-spinner Graeme Swann.