No, when the water temperature reaches 100°C and there is no rise in temperature, it does not necessarily indicate thermal equilibrium. Thermal equilibrium occurs when two or more objects or systems reach the same temperature and there is no net heat transfer between them. In the case of water reaching 100°C, it signifies that the water has reached its boiling point, where it undergoes a phase change from a liquid to a gas (water vapor).
During the phase change from liquid to gas, the temperature of the water remains constant at 100°C until all the liquid water has converted to water vapor. This constant temperature during a phase change is known as the "latent heat" region. The energy being supplied to the water is being utilized to break the intermolecular bonds between water molecules rather than increasing the temperature. Once all the water has vaporized, further heating would result in a temperature rise again.
So, when the water temperature remains at 100°C without any rise, it indicates that the water is undergoing a phase change rather than being in thermal equilibrium with its surroundings.