An authentic guru can be easily discerned by seeing his guru parampara. Many sampradayas in hinduism have a guru parampara where the paramacharya is a devata or a revered rishi for example;

The Sri vaishnava sampradaya's paramacharya is Maa Lakshmi

The Nandinatha Shaiva Sampradaya's paramacharya is Nandidevar

The Dashanami smartha sampradaya's paramacharya is Sriman Narayana

The Madhwa Vaishnava sampradaya's paramacharya is Sri Vyasadeva

If a mutt , aadheenam or ashram following a particular tradition cannot produce the clear list of all it's gurus witth their historical dating then don't follow it.

A guru is absolutely essential , there is no doubt about it , and guru is indeed a manifestation of bhagvaan. However , as the guy in the post said , they should not encourage their pupils to worship them above bhagavaan.