But if I use high impedance and probe - why there wouldn’t be reflection back to the line?
if you use high impedance probe, your CAN line needs to be terminated. It is already terminated if it's connected to another receiver with 120 Ω input. If you're needs to measure line which is not connected to anything, then you're needs to put 120 Ω termination resistor with high impedance input or use oscilloscope with proper input impedance.
If you're want to measure 120 Ω line which is already terminated by another receiver, then oscilloscope with 50 Ω input is not suitable for you, because when you tap your 50 Ω probe to already terminated 120 Ω line, the point where you tap the probe will have 35 Ω impedance and as result it leads to wave reflection.
Also note, when you use high impedance probe, it has bandwidth limitation. So, if you want to see more high frequency details, it's better to use 50 Ω input with proper attenuator. But it will works when your line is not connected to anything else, this way will not work if your line is already terminated with another receiver. When the line is already terminated, the only way is to use high impedance probe.
In simple words, if you're needs to catch transmission between devices, then just use high impedance probe. If you're want to measure parameters of device output which is not connected to anything else, then you're needs to use proper terminator resistor with high impedance oscilloscope input or use proper attenuator with 50 Ω oscilloscope input.
The main idea is that endpoint of line needs to have proper 120 Ω resistance (for 120 Ω line) which include resistance of the oscilloscope probe. If oscilloscope probe has too low resistance you can add resistor in series to get actual 120 Ω. If oscilloscope probe has too high resistance, you're needs to add resistor in parallel to get actual 120 Ω. This is how it works.
Also note that resistor should not be inductive. Some kind of resistors can have too high inductance and can't be used. Use RF grade resistors or at least film type with no winding for such purposes. SMD resistors bellow 200 Ω usually works ok up to 50-100 MHz and even more.