If I read correctly - does this mean that before RingCT was introduced in Jan 2017, all transactions on Monero's blockchain are completely traceable? I.e. ring signatures did not work to hide the transaction amounts or sender.
No. Please take a concerted effort to read both responses:
Basically the portion that was traceable was transactions where the decoy inputs were deducible, i.e., the decoy inputs were provably spent and thus were impracticable decoy inputs. As a result, you could easily single out the real input.