How did scientists arrive at this data?

• An international team of scientists led by researchers from the University of Cambridge, began by investigating an ice-sheet-covered region called the Ultimis Scopili. Previous techniques of the past had not produced any results, and so this time they opted for a different technique.
• They used spacecraft laser-altimeter measurements from NASA's Mars Global Surveyor satellite to map the topography and shape of the upper surface of the ice cap. They examined the surface topography from the same area where Mars Express made its radar measurements, and found a 6.2-mile to 9.3-mile-long (10 to 15 km) surface undulation.
• This ice surface consisted of depressions and corresponding raised areas, and resembled the undulations noticed in ice sheets above subglacial lakes on earth.
• To determine if this undulation could be the result of subglacial water, the team introduced their computer model of a Martian ice sheet a patch of reduced bed friction where water would allow the ice flow to speed up. The researchers also adjusted the amount of geothermal heat in the simulation. These simulations caused undulations in the computer-modeled ice surface, which were similar in size and shape to the southern polar ice cap on Mars.?
• A combination of the results from this simulation, the new topography observations of the ice cap, and the 2018 radar results point toward the existence of subglacial water beneath the southern polar ice cap, with deeper implications for the geology of the red planet.

The team thinks their results indicate that the geothermal heat needed to account for the subglacial water may come from magmatic activity that has occurred relatively recently in the subsurface of Mars.

Why everyone initially was not onboard with the idea?
Mars, like Earth, has thick water ice caps at both poles, roughly equivalent to the Greenland Ice Sheet. However, unlike Earth's ice sheets, which have water-filled channels and large subglacial lakes, the polar ice caps on Mars were thought to be entirely frozen.
The European Space Agency's Mars Express orbiter in 2018 challenged this assumption. Its radar called MARSIS that can see through Mars' southern ice cap, and found many dips and rises on the surface of the ice cap covering the south pole of Mars, implying there is liquid water underneath. But not all scientists were convinced at that time. Some of them thought the strange radar signal measured by the spacecraft might be explained by, for example, some sort of dry material below the ice caps.

How do the findings impact theories trying to prove life on Mars?
The researchers have noted that In order to be liquid at such cold temperatures, the water beneath the South Pole might need to be really salty. This would make it difficult for any microbial life to inhabit it.
This, however, gives hope that perhaps there existed habitable environments before when the climate was less unforgiving.
?