You would need to get a CDMA phone to work in Marathon (can check this on Koodo coverage maps)
CDMA is basically an older phone that does not have a SIM card, so the newer smart phones would not work there, but you would be able to get service.
As to what you were told about coverage - Koodo agents do not know when or where towers are being built - this would be decided way further up, and the agents at Koodo support would have a general knowledge but nothing definite
I have been a koodo customer for yrs now and keep getting told different dates when tbaytel will be finally told by fed regulators to open up our area. I was talking to support about 3 months ago and they told me that we would finally be able to get a marathon phone number. The number I have doesn't hurt me because I have no long distance charges, but if someone needs to call me it's always long distance for them