Actually kishi said hiruzen is the strongest. It has never even been hinted that hashirama was the strongest except by madara.
hashirama>hiruzen
hiruzens hype-lets start out with hiruzens hype. he was stated to be the strongest kage, the professor, and mastered all of konohas jutsu at one point.
hashiramas hype- now for hashiramas hype. hashiramas strength was stated to be a myth, a fairy tail like the sage of six paths. he was also called the strongest shinobi. it was also stated that hashiramas might was so vast that no shinobi from the current era could stand up to his power. that means that hashirama>every other shinobi except edo madara.
explaining the hype-we will get back to hashiramas hype. hiruzen was stated to be the strongest kage. nobody at the time from when hiruzen was getting all his fame(which was his prime) knew about or believed in hashiramas strength. hashiramas strength was considered a myth.
now lets define myth:
a traditional or legendary story, usually concerning some being or hero or event, with or without a determinable basis of fact or a natural explanation, especially one that is concerned with deities or demigods and explains some practice, rite, or phenomenon of nature.
so the people that tried to tell the future generations about hashiramas strength were dismissed because nobody thought a shinobi could be so strong. this is supported by the manga because hashiramas strength was considered a myth, a fairy tail like rikudo sennin. does everyone believe in the sage. nope.
Feats- now lets get to their feats. hiruzen is severely outclassed in this department. hashirama defeated ems madara with the kyuubi. ems madara by himself is one of the strongest characters in the narutoverse. combine this with the fact that he had the kyuubi by his side which is the strongest tailed beast, then you can see why hashiramas strength was considered a myth. hiruzen had to pull out reaper death seal on a nerfed edo hashirama and edo tobirama who were stated by orochimaru to be toying with hiruzen. while hiruzen very well may be much stronger in his prime, his hype and feats put him nowhere near hashiramas level.
databooks- databooks have been wrong on multiple occasions like saying orochimaru invented edo tensei and saying naruto and sakura were equals. the info in these can be faulty and it is outdated. how long has it been since kishi has released a databook? not recently.
conclusion- people from the future generations that heard of hashiramas strength considered his strength a myth. people believed that hiuzen was the strongest didnt believe in hashiramas strength. they reasoned that hiruzen was the strongest because that was what they saw and hiruzen>hashirama is what seemed reasonable to them because hashiramas strength seemed to exaggerated.
hashirama>hiruzen
