Yeahh but like how did she specifically know 40 years tho? That's the confusion am facing.
That probably was part of the TLDR they gave at the start of the chapter since it start with "To sum it up", so they don't repeat all over again the same thing we know about already, with mc mentioning that part to her in the full story, which likely went "i got isekai in the year 2000, spent some years there, saved the world and then got betrayed and killed, and now am in this body a bunch of years later somehow"
Unless you wanted an extra page with mc overexplaining something we as reader already know
edit: checking out the WN just in case (though this isn't an adaptation and manga is the og work),
the 40 years is mentioned there in the TLDR, but manga omitted that part even in the raws lol, tho thats probably why the author decided to do a WN version for this work too, for those extra details i guess (even if the author said that the novelization won't affect the manga)