Out of all the member's 4Runners here that we've analysed, diagnosed, theorized, argued and socialized over, I have yet to view a conversation regarding a corroding frame... "issue". Additionally, if that was a serious problem I'm sure that Toyota (being extremely image aware), would certainly have moved the hose to a different location; reputation being of the uttermost concern! Since the Tacoma in the link is a 2015, I would gamble on the theory that this rust was only superficial or, a sealer/coating defect 'on that particular truck'; ...rust on the frame in just a couple months?? ..."Something went horribly wrong there!"
Yes, the elbow points right down onto the frame. My truck was originally from New York and is rusty to begin with but I didn't see anything excessive at the dumping point that would lead me to believe that it's a large issue. In 15 years that could be a different story.