When did handguns get to be "platforms?"
I don't know.
I don't think so. I think it's just plain wrong.
Originally, platforms were things to stand on, walk on, put things on, mount things on, and so forth.
"Weapons platforms" were and are things used to mount weapons
on. This usage goes back a while.
Some decades ago, the term "platform" was adopted to define a computer software and hardware architecture.
On a related subject, one can now acquire software systems known as trading platforms
to facilitate the buying and selling of stocks and bonds.
I could see how someone might call an M-16 on which a rocket grenade launcher was mounted a "platform".
But a 1911 "platform"?--no, not for me.
I wonder how many yuppies have 1911 pistols....