The internal of the oscilloscope has an amplifier limitation. They called it dynamic range. If you use your scope pass the limitation, your reading will no longer be accurate. The linear amplifier will start to become nonlinear.
If you look at any block design of the oscilloscope, you will notice the input amplifier or preamplifier. You will not see filter block in front of it. The input signal is too small before it can be process by a filter. After you amplified the signal then you can use a filter. So the limitation is the preamplifier not a filter. When the o-scope give you a spec of 100 Mhz, 3dB. You can be sure it's referring to the preamplifier.