This is simply not true. Environmental Impact Reports are very expensive to conduct. Trust me, my wife does them for a living.
However, I do think in this case an EIR would show a significant positive impact in the traffic section. I stayed in Park City a total of 5 days and on three of them I drove all the way around to go up LCC / BCC - and we were not alone. As for the water shed part, well, that's complicated and speculation is pointless.
I don't think this is going to come down to an EIR, however. It's going to be a cost benefit analysis, factoring in the legal process that will definitely occur. Ski lifts and a corresponding (and ongoing) shuttle bus system are not cheap. Not to mention additional terrain for the various ski patrols to control.
The SkiLink site in the FAQ section claims no additional terrain is planned. The One Wasatch site also has a FAQ section, but the question of additional terrain is not mentioned. I think the idea is to just move people from one resort to another using 6 lifts, but not to expand terrain. http://onewasatch.com/?page_id=105#toggle-id-7