You'd never rule out anything in something as diverse as the Star Wars galaxy- but first of all a single species pirate clan is very rare indeed, and secondly the White Fortress, so the legend goes, has been used by all sorts throughout the ages. That cant be tied to a single radiation-resistant type.
Saar will field one of your points there.
"There's no suit known that could protect from that level of radiation," says Saar. "You'd need heavy industrial equipment planet-side to build a powerful protection shield. But anyone you sent would be dead before they could build it, even if they could get the electronics working.
"Assuming that planet is what it seems, of course."
"Perhaps there is something in the atmosphere that blocks the radiation from getting to the planet's surface? We keep saying that it is in close proximity to a high-radiation star, but people have almost certainly visited. Since no suit would save them and the only technologically feasible alternative, a shield generator, is a deadly catch-22, what else is there? You fly in on a shielded ship and land on safe ground."
"No ship capable of landing has a powerful enough shield. But you are correct- there must be something that blocks the radiation.
"Once in a while... very rarely... a ship in that far out place, so far from civilization, comes to this system and notes something odd. The planet, that others have seen and shied away from, is habitable after all. They are curious. They investigate. And they find the White Fortress is there.
"There is something strange about the orbit of the start and some dense stellar objects that its orbit lines up with. In fact, the planet is only deadly for ten-year periods. Then, the strange orbits synchronise up, and the planet is habitable for some few years- before the inevitable doom comes again."