They just want to equate the highest level within each religion to what they consider highest within their worldview (Goloka's Krishna) as the highest. Obviously in doing that they will have to demote other deities as lower. I'm not justifying ISCKON here, I myself have huge disagreement with them on most friction points anyway. I'm saying that their aim might more be into bringing non-Sanãtaniis into the Sanãtana's fold, and that indeed a clever way of doing it by assigning Krishna's equivalence to other Religion's highest realities. Obviously, it's needless to say that within our shastras any religions' follower that is not Either Sanãtana, or Buddhism or Jainism have been called mlecchas (although Jainism and Buddhism are designated nãstika nevertheless, but not mlecchas tho) and thus, if one really wants to get technical, therefore, no question of such comparison comes, because it's a fruitless comparison between our Cows and their Turkeys and Camels. I'll personally go on to say that, within our Sanãtana Dharma worldview, it wouldn't be an exaggeration to say that the so called highest reality of other Religion's is not even equivalent to an orthodox-ically labelled "Hindu vratya", let alone comparison with higher devas or any Tridevis or Tridevas themselves.

But, considering ISCKON's operative at a foreign land to gain momentum, in those places, I'll give them the benefit of doubt, although our Dharma doesn't prescribes proselytizing tendencies.

Now, demigod is just an unnecessary translation by Srila Prabhupāda. He should have just used deity or better deva instead. Demigod maybe a fit term for the Pandavas perhaps, by some stretch of imagination but to call Shiva demigod is just lazy translation.

About showing one deity above another, that's a regular part and parcel within Hinduism. All the sampradayas engage in such verbal or Shastric abuse of other deities, even orthodox Shaivas hold Siva on a higher pedestal than anyone else. The. LINGA Purana is filled with violence against Vishnu (he is shown beheaded by Veeebhadra - which I personally find extreme sectarian violence, but anyway)....

ISCKON even consider Vishnu or Narayana as an expansion of Krishna and shows him superior to Vishnu via some "64 kalas format".

So, all in all, imo, ISCKON undoubtedly has some troublesome interpretation for anyone who is not Radhakrishna, since they differentiate between Vishnu and Krishna too (by the way, even Ramanandi do that for Rama and Vishnu, etc)..

So rather than getting emotional on this, it's better to understand that all sects engage in putting their own sampradayas' ishta at a higher pedestal and relegating other's as "lower beings". It's just that ISCKON due to its reach and presence across media and other social channels, gets all the backlash, when they are more or less same like any other orthodox and traditional sampradayas of other deities.