How is it possible that Ragnarr Loðbrók is considered to maybe be a mythological person who didn't really exist, but his children were definitely real?

People claiming legendary figures as ancestors for purposes of clout never gets old. More can always be said on the matter, if anyone would like to have a go at He Who Wears Fuzzy Pants; for the meantime, OP, we do have some previous threads that may be to your interest:

u/sagathain examines the man wearing fuzzy pants and his sons;
u/y_sengaku looks at the man wearing fuzzy pants as a figure;
and u/Platypuskeeper observes what we know of he who wears fuzzy pants.