Just ... No. Read the history. At least read Wikipedia.

"Staged combustion (Замкнутая схема) was first proposed by Alexey Isaev in 1949. The first staged combustion engine was the S1.5400 (11D33) used in the Soviet planetary rocket, designed by Melnikov, a former assistant to Isaev.[1] About the same time (1959), Nikolai Kuznetsov began work on the closed cycle engine NK-9 for Korolev's orbital ICBM, GR-1. Kuznetsov later evolved that design into the NK-15 and NK-33 engines for the unsuccessful Lunar N1 rocket. The non-cryogenic N2O4/UDMH engine RD-253 using staged combustion was developed by Valentin Glushko circa 1963 for the Proton rocket."

And way down at the bottom of a long engine list spanning more than half a century...

"Raptor—SpaceX LCH4/LOX engine in development, first flown in 2019[19][20]"

https://en.wikipedia.org/wiki/Staged_combustion_cycle

> The first flight test of a full-flow staged-combustion engine occurred on 25 July 2019 when SpaceX flew their Raptor methalox FFSC engine at their South Texas Launch Site.[8]

(Yes, there are others listed, but SpaceX was first to actually fly something.)

thank you, the part about invention I care about is not ideation but actual reduction to practice.

The "long engine list" is only 3 deep, none of which besides the raptor made it past the test stand. What matters is full-flow-ness, not staged-ness, which means running neither oxidizer- or fuel- rich, for maximum efficiency. And in any case the russian engine was N2O2/UDMH and the american prior art was hydralox, so it's not like spacex didn't have to do a TON of figuring out to get a methalox FF engine up and running.

He said "full-flow engines". None of those that you cited were full-flow.

different fuels from the raptor, though, so it's not like spacex didn't have to figure a ton of stuff out anyways, and also it never flew.

I'm not trying to detract from the monumental achievement SpaceX's engineers have made with the development of the Raptor, but that development work didn't take place in a vacuum. Unfortunately, I don't have a credible source for this, but I have seen multiple reports that the Raptor program was explicitly a continuation of the IPD program, with SpaceX receiving technical data, hardware, and even hiring engineers who worked on it.

That the IPD was hydrolox and didn't proceed to the point of a full engine, let alone a flight is besides the point.

What you said was: "They kind of did invent full-flow engines though", which is completely false.