Warning! SPOILERS for Obi-Wan Kenobi episode 2.
Obi-Wan Kenobi episode 2 revealed Reva knew Darth Vader's greatest secret: that he is really Anakin Skywalker. Emperor Palpatine kept Darth Vader's true identity in Star Wars a closely guarded secret, in part because it amused him to do so and in part because he believed a mysterious, monstrous being was far more terrifying. Only a handful of people knew the truth that Darth Vader was really Anakin Skywalker: key figures in the Empire and leaders in the Rebel Alliance who'd learned from the Jedi.
According to James Luceno's novel Tarkin, rumors swirled about Darth Vader's origin. Some claimed he was a counterpart to the Separatist leader General Grievous, held in reserve by Chancellor Palpatine during the Clone Wars in case the Jedi betrayed him and brought out after Star Wars' Order 66. "Many believed that the Emperor’s willingness to grant so much authority to such a being heralded the shape of things to come," Luceno writes, "for it was beyond dispute that Vader was the Empire’s first terror weapon." Given this is the case, many viewers were surprised when Obi-Wan Kenobi episode 2 saw Reva reveal she knew Darth Vader's true identity. Sensing Obi-Wan's shock when she mentioned Lord Vader, Reva confirmed Anakin Skywalker was still alive - also showing she knew what had happened on Mustafar.
The clear implication is that all the Inquisitors knew Darth Vader's true origins, which is indeed something of a surprise. Still, it does make sense from an in-universe perspective. Inquisitors were not Sith, but they served the Sith's purposes, trained in many dark side arts to hunt Jedi across the galaxy. They worked closely with Darth Vader himself, and they'd presumably have access to a top secret database, allowing them to plot the movements of all known Order 66 survivors. Obi-Wan Kenobi was a high-level target, one believed to be the last surviving Jedi Master, and it's reasonable to assume they were given information on his last known movements. That would inevitably include his duel with Anakin Skywalker on Mustafar.
Reva isn't just bragging when she name-drops Anakin Skywalker, of course. She's taunting Obi-Wan, hoping to provoke the Jedi Master into doing something stupid. It's clear Reva is unique even among Inquisitors, because she possesses unusually potent empathic abilities, allowing her to sense the emotions of her targets. Even so, she shouldn't have been able to get a read on a Jedi Master quite so easily, indicating just how out-of-practice Obi-Wan has become at keeping his emotions in check.
Obi-Wan has also unwittingly revealed a little about how he has hidden, because his ignorance of Darth Vader indicates he has been isolated - most likely on the Outer Rim. Reva can count her attempt to flush Obi-Wan out of hiding as a success. She's proven a connection between Obi-Wan and Star Wars' Senator Bail Organa, one that can be used against him. Now the Jedi Master has given her a hint of where he's been hiding up till now. The rest of the Obi-Wan Kenobi Disney+ TV show must see the Jedi Master try to get the Empire off his trail again.
