Took me a while to figure out what you were referring to, but yes, if you use the variable $posts as shown in that first code example it will overwrite the entire global posts array that WP uses to loop over the main query.
I’ll mark this topic for the developer to take a look at when he has time. The documentation should probably be altered to avoid referring to other global values as well.
The problem that you encountered could also be corrected (I think) by using wp_reset_query() in place of wp_reset_postdata() https://codex.wordpress.org/Function_Reference/wp_reset_query
