Electret microphone RC filter design contradiction among information sources
First, the youtube source shows an 1st order active filter, maxim source shows 2nd order active filter and last source shows first order passive filter. That's three different designs.

A 1st order active filter can be implemented either in the negative feedback branch of the op amp (as in youtube source), this does not need to be grounded.

Or it can be done as a classical RC circuit followed by impedance converter or amplifier. Here, either the R goes to ground (highpass) or the C goes to ground (lowpass)

If you combine these two approaches you get a 2nd order filter. So, wether "the filter needs to be grounded", as you ask, depends strongly on where its implemented.