I've found it's worth treating some Blackbird writings with scepticism even when from seemingly authoritative sources.
A significant percentage of air entering the inlet bypassed the engine through ducts and traveled directly to the afterburner
No air bypassed the engine and went straight to the ab. Most of the ab air,80%, went through the whole engine. A small amount, 20% which was required to make the compressor function properly at high inlet temps, was bled from the 4th compressor stage and then to the ab.
Sources: US Patent 3,344,606 which is the bleed air patent, F-12 Flight manual Fig 1-1G for example. Also the popular colored airflow diagrams in a previous post which don't show any engine bypass air going to the ab.
engineers described the powerplant as a turbo-ramjet
The engineers who designed and built the engine(P&W), those who designed and built the installation (Lockheed) and those that serviced and maintained it called it a bleed bypass turbojet with afterburner, eg see Flight manual. This knowledge alone sets the alarm bells off when you see the word ramjet. It was, after all, a turbojet with a bit more compressor bleed than usual.
One of many good sources: 'F-12 series aircraft propulsion system performance and development' by D. Campbell, the Lockheed engineer who also took out US Patent 3,477,455 for the inlet. Incidentally one of the drawings in the patent is the basis for the popular colored airflow diagrams which, in fact, are taken from the flight manual.