Most of them are legitimate.
This game is not a golf game. It is a golf themed computer game. Most of those low scores are shot by game players who take advantage of the unlimited play aspects of the tournament. They quit the round and start over again when they get a bad shot or a bad score on a hole. Thus, by the time they actually post an 18 hole score they may have played the first hole 1000 times, the second hole 950 times, the third hole 900 times, etc. until they finally got to hit 50 or 51 shots in a row that yielded the low score posted.
If tournaments were single play you would still see the occasional great player who hit it the first round but there wouldn't be nearly the volume of low scores you are looking at.
They will never be single play,though. Those players are investing a lot of credits (money) in golf balls starting those tournament rounds over and over. The unlimited play aspect of tournaments must be a huge contributor to WGT's bottom line.
Some of those tournaments have conditions that are less difficult than those players tiers, too. A Legend or Tour Legend caliber player, playing from forward tees with little wind, playing with the best equipment, will be throwing darts with wedges for eagle the entire round.