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The "decoupled" in this scenario would probably refer to the series capacitor that blocks DC, although it should be "coupling capacitor". This is used commonly in audio amplifier circuits to allow one stages input to be independently biased from the previous stages DC output level. In the "tape earphone" circuit C24 performs this function. I'm not sure about the "tape input" bit though, as the tape recorders input circuit should have a coupling capacitor in place anyway (maybe it means this, but it does say "prior to..") It looks just like a RC attenuating circuit to me.

Yes, as long as you supply the opamp with the same voltage as the FPGA pins (e.g. 3.3V, 2.5V, etc) rather than the 5V shown. You should be able to use any opamp for the comparator function.