Questions about a MG811 module

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Hello,

I want to do a CO2 monitor based on a MG811 sensor and an Arduino UNO card.
I found a module but the schematic seems "weird" from my point of view.
www.dfrobot.com/image/data/SEN0159/CO2%20Sensor%20V1%20SCH.pdf

I see a LM358, but according to the MG811's datasheet (image.dfrobot.com/image/data/SEN0159/CO2b%20MG811%20datasheet.pdf) the impedance of the amplifier should be about 100G and the input current below 1pA. These values do not fit with the 20nA and 100M of the LM358.
After some searches, I found a better circuit (http://sandboxelectronics.com/?p=147) which uses a LMC662 (closer to the sensor requirements).
But I still have some questions:
- why a resistor is added just before the non-inverting input? The LMC662 input bias current is very low (2fA), so why? And why 1k?
- why a such amplifier circuit? It looks like an old NAB filter. Is it used to filter anything from heater power supply (I doubt it since it works at about 450kHz) ?

Thx for any input.

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Instead of a link to a PDF file, here's the circuit as attachment.