Water can absorb enormous amount of energy. And releases huge amounts as it evaporates. And the lake is in contact with massive land surface that also absorbs ton of energy.
The deeper parts of Seneca lake are isothermal with regional ground temperature, so there's clearly no problem with heat dissipation
I do suspect that this is a surface temperature problem. Lakes form thermal gradients pretty easily so if they are only heating the top layer this power plant could noticeably warm the water even if it is producing nowhere near enough heat to warm the entire lake.
I can't argue against the take that this whole thing is just killing the planet for a profit.
The thermal conductivity of water makes that even a temperature a few degrees cooler than the body temperature feels refreshing, which is not the case right now according to the witnesses.
Notice they mentioned "surface temperature". Many lakes are naturally stratified in summer. Especially lakes deep for their size. https://en.m.wikipedia.org/wiki/Lake_stratification
Indeed, Seneca lake is very deep, and naturally thermally stratified in summer:
> Because of Seneca Lake's great depth its temperature remains a near-constant 39 °F (4 °C).[3] In summer the top 10 to 15 feet (3.0 to 4.6 m) warms to 70–80 °F (21–27 °C)
https://en.m.wikipedia.org/wiki/Seneca_Lake_(New_York)
I think crypto mining is a huge waste of energy. But this article has an ax to grind. A simpler explanation for warm surface temps is a warmer than usual summer.
Indeed, NY state has had two major heat waves already this year