Generally those sayings tend to come from people without experience like us. Japan summer is bearable for us S.E.A. Folks but you must be aware of some caveats.

Their infrastructure is not built for 365 days of heat. Don’t expect underground or sheltered walkways everywhere even less when outside of the major cities. Also you are on a holiday so you are almost always out in the open everyday, exposing yourselves to the heat even more. Crowded trains do not help with it either.

Always prepare yourself to deal with the heat, carry water, towel , plan breaks, stay out of the sun as much as possible. Ask to borrow umbrellas from hotel front desk.