The issue with hot water and carboys is thermo expansion associated with the temperature change. In the glass industry it’s called thermo shock. For a typical wine bottle and most glass containers it’s recommended the temperature delta should not exceed 70 deg F.
For carboys it’s a crap shoot as some are from the early 1900’s to modern day and all have been manufactured to different specifications and the delta 70 deg may not apply as they are specialty type containers. You can use hot water to clean carboys as long as to avoid large temperature swings in the water, you might want to rinse in warm water to preheat the glass and gradually increase the temperature of the entire carboy.
Someone mentioned the bottom being the thickest area and it should be the strongest there, actually sort of. The bottom of the carboy has the most stress and needs to be thicker/stronger so it doesn’t break. Areas of high stress need to be thicker/stronger. Impact, internal pressure/vacuum, top load (stacking) and thermo shock all multiply the stress and when the stress exceeds the glass strength it breaks. So keep the stress to a minimum.
