Why? Because a bunch of people who have mindlessly accepted a "known fact" keep mindlessly stating and restating the thing "everyone knows" that not a single one of them has actually tried or investigated ..... kind of like you have just done.
The Model 7 was also made in 243 and 308W. Cartridges that are considerably larger diameter than the 223, meaning the barrel is thick enough to withstand a larger diameter case, then the even thicker walls left behind after creating the 223 chamber will be even stronger. I think its safe.
The whole 223 vs 556 thing has been WILDLY overblown and just repeated endlessly by people who have never researched the issue or actually tried to see if it is true. People who know nothing about firearms design or manufacture .... facts which don't seem to prevent them talking sh!t endlessly.