I used to be a very frequent Biz class customer. I had Gold with SQ and BA. In those days, I travelled biz and could understand the brace position.
Since I retired I am a holidays flying customer.
I cannot imagine how to adopt the brace position in any economy flight. It is not physically possible for me at over 6 feet tall.
Basically, I infer from that, that it is OK according to the safety gurus to just lie to customers about safety, just as they lie about Customer Service.
I did once ask the nice people at the Belgrano, but was not persistent enough to get a reply. (A quick plug for their medics - they are brilliant).
Even though I know that I am being told rubbish, wrt the brace position, I have resolved to carefully listen to each and every safety briefing. (I used to read the paper/a book, but a BMD steward told me off once in very fierce terms, so now I listen and watch on every flight).