Yep, you're right...you will be corrected. This is bunk. If it were true, headphones wouldn't work. If the sound is stronger 15 feet away, it's because of boundary reflections in the room. Unfortunately, a lot of pro audio people still believe this nonsense and keep saying it as gospel. So it's not your fault.
You are also wrong that 115 cabs aren't great for smaller venues. It's a matter of taste. I often use 115 cabs on gigs big and small, and they are always great (at least mine are). What's not so great, at least to me, is combining them with a 410. The 410 drowns out the 115, plus it can cause phasing issues. A lot of people do it and like it, but I have always hated combining speakers like that. Either 10"s or 15"s for me, never both.