By the 1300’s Vikings had travelled through out Europe, no doubt spreading tales of discoveries far and wide. That some monk heard and recorded such a tale about lands far to the west is hardly astonishing. In preprinting press days, no surprise that it never went anywhere.
As for “giants,” native Americans were of stature equal to the Vikings and taller than most southern europeans.
Except those did not build stone slab buildings. Sounds like pyramids/temples if anything, but then the Vikings would've made it all the way to Mayan territory.