That's not remotely true, I'm not sure what gave you that idea. Bitcoin at its core is based around asymmetric encryption, which allows only 1 party to send money but everyone to verify it. By sharing a public key (a bitcoin address) you can see the funds, but can not spend it. In fact, right from the start bitcoin client even included the ability to sign and verify messages from addresses to prove ownership. Right now I could give you the bip44 (public key) of my wallet and you could see every transaction I have received, sent (and all future ones, from that wallet). But you'll have no ability to spend my money. Although I'm not going to do that for the same reason most people won't: privacy.
Anyway, there's been proof of solvency schemes for a long times, it's just most exchanges and services (with a few notable exceptions) haven't seen a business case in implementing it.