Keep in mind that indoor use rollers can upload to Strava as if they were a properly done activity. This implies that downhill segments are being taken easily by that means, since they do not account for the risk. Also, aiming for downhill segments is pretty much setting yourself for a crash, people have already died because of that.
In any case, even if the KOM is genuine, your bike might not be the same as the people riding faster, who may be in expensive machines with top notch wheels, tyres, brakes and a geometry more suited for descent. In the case of going uphill it is the same, people may have a better suited machine, but the risk of trying to match them is considerably smaller. Also the rider can be short and heavy, with a lower centre of gravity and whatnot. Some may take risks and go into the opposite lane, or a certain KOM may have been taken with closed road during an event.
Gauging the speed at which you can take on a turn is tricky, even your tyre pressure can influence that. Try to get information on the best descending techniques and practice if you really want to get better. Your equipment, position on the bike, and weather will influence what is the maximum speed you can achieve on a given turn, and with practice you will get closer to that maximum. But let me remark that it is not a good idea to go for downhill KOMs. As the velominati rules put it, your confidence going downhill increases until it drops all of a sudden.