JUICE probe will be 1st to use gravity of Earth and moon to slingshot to Jupiter
In short this 'double gravity assist' to propel JUICE towards Jupiter, has not been done before, so that is why it is a first.
TL;DR:
Unfortunately the article you link skirts the basic details, but a quick google results in:
JUICE ...on a mission to make detailed observations of the giant gas planet and its three large, ocean-bearing moons – Ganymede, Callisto and Europa.
Why does it take so long to get to Jupiter? Well, the short answer is that its less to do with the distance between Earth and Jupiter and more to do with fighting the Sun’s massive gravitational pull as you venture outwards through the Solar System.
...they make use of ‘gravity-assist’ (or ‘flyby’) manoeuvres to gain energy by swinging through the strong gravitational fields of various planets on the way.
Shooting for the Moon
Juice’s first boost will come from its home planet, when it returns to Earth in August 2024, more than one year after launch.
In fact, in a first-of-its-kind flyby, Juice will first pass by the Moon to give it an extra kick and make the flyby of Earth that takes place 1.5 days later even more effective.
But even with this extra boost, to get the most out of the gravity assist, Juice has to arrive at the Earth-Moon system at precisely the correct time, at the correct speed and travelling in the correct direction.
That’s where today’s (2023) manoeuvre comes in.
On 17 November, at 16:10 (CET) (2023), Juice’s main engine performed a burn that lasted roughly 43 minutes.
“This manoeuvre used up roughly 363 kg of fuel – or almost exactly 10% of the 3650 kg of fuel that Juice launched with,” says Julia Schwartz, Flight Dynamics Engineer at ESA’s ESOC mission control centre in Germany.
“It was the first part of a two-part manoeuvre to put Juice on the correct trajectory for next summer’s encounter with Earth and the Moon. This first burn did 95% of the work, changing Juice’s velocity by almost 200 m/s,” adds Julia.
“Juice is one of the heaviest interplanetary spacecraft ever launched, with a total mass of around 6000 kg, so it took a lot of force and a lot of fuel to achieve this.”
https://www.esa.int/Enabling_Support/Operations/Juice_burns_hard_towards_first-ever_lunar-Earth_flyby
This burn is one of two necessary to put JUICE on the right trajectory for the Earth-moon gravity assist, the first in the series of assists, which will occur in Aug. 2024.
"This first burn did 95% of the work, changing JUICE’s velocity by almost 200 meters per second [656 feet per second]," said Schwartz. "JUICE is one of the heaviest interplanetary spacecraft ever launched, with a total mass of around 6,000 kilograms [13,228 pounds], so it took a lot of force and a lot of fuel to achieve this."
https://www.space.com/esa-juice-spacecraft-executes-major-maneuver
"A lunar-Earth flyby has never been attempted before," says Angela Dietz. "There are risks, but all of Juice's onboard systems have been rigorously tested, and we are well prepared." She added, "Due to the low altitudes, large velocity changes and strict navigation requirements involved, the flight control team needs to be ready to react at a moment's notice."
https://www.spacedaily.com/reports/Juice_Prepares_for_Historic_Double_Flyby_of_Moon_and_Earth_999.html
https://universemagazine.com/en/take-a-shortcut-to-venus-juice-prepares-for-a-historic-double-flyby-of-the-moon-and-earth/
