The problem is not Garou's copy, but Saitama's growth.
Let's supose Saitama's full power is 100. Garou copies it and has now a strength of 100. Then Saitama instantly becomes stronger and his power is of 1000 (don't take the numbers as a multiplier which is or May by any mean be accepted, just as an example to understand the difference). Garou copies the power of 1000, then Saitama's power instantly increases to 100000. Garou copies the power of 100000, then Saitama's power instantly increases to 1000000000. The gap is becoming higher and higher, so eventually the Gap'll be so high Garou'll be one-shotted before he could copy such power. That's what the graphic means:
You can perfectly watch that every time Garou copies Saitama's power, the curve of Saitama's power increases: he's becoming far stronger in far less time
Don't even try to argue that what I said means Garou can copy strength of a Googol of Joules, please. I clearly didn't mean that and you know it