Keep in mind that Police generally carry those Glocks in a duty holster and that is different than tucking it away in a concealed IWB holster that you can't see. Re-holstering striker weapons that do not have a manual safety require special care to make sure that articles of clothing do not start in the holster with the gun. We see this at the IDPA matches where participants get help from the range officer when clothing starts to get in the holster during final re-holstering. Fortunately, IDPA protocol requires clearing of the weapon as each shooter finishes his turn. Glock can be carried in a kydex IWB snap on clip holster and following use the holster could be withdrawn to permit holstering with both hands in front of the operator and the holster with gun encased could then be put back(as a unit with gun in holster) on the belt. Of course, if the owner has visual of the holstering, that would be enough to ensure safety.
The likelihood of actual armed confrontation is very small, but there is real daily risk from mishap during gun handling. The old LC9 with the long trigger pull did not need a thumb safety, but the new striker replacement benefits greatly from the thumb safety IMO. To answer your question, YES the LC9s is safe to carry chambered with the thumb safety on. Make sure that your thumb safety is positive and does not flop out of safe position as some I have seen do. If your safety is not positive, contact Ruger, they will be glad to fix it for you.