Is the Guru in that list not the teacher! Its not the preacher! Unfortunately people confuse between the two
Kannadasan talks beautifully in his Arthamulla hindu madam
Child only know Mother (Madha)
Thai kaatinal Thandai
The mother has to let the child know who the father is (Pitha)
The man/woman who imparts Knowledge or opens their eyes is Guru...
And then comes God
Sri
If I can stop one heart from breaking,I shall not live in vain; If I can ease one life the aching,Or cool one pain, Or help one fainting robin, Into his nest again, I shall not live in vain. Emily Dickinson
The definition of Guru is only one, as in learning from everything.
A physical guru can be for a period of time when you learn and outgrow, then you move on.
The concept is to keep learning, thats the necessity of having a guru. Anubuthi: "Guruvai varuvai arulvai guhane", ultimately the guha is in the depth of our own self, where we as a different entity will recognize the beauty of humility in front of nature when we see that the Guru is all around us.
Anubuthi: "Yaanaga iruntha ennai vizhungi thaanai nilai nindra thar parame..."
Which is what happens when you accept a guru from everywhere.
You know a guru when you see one, not necessarily in a saffron robe :-)
Now in same context, if you read the matha pitha stuff, it makes a big sense, when actually you should be seeing God in everything as a teacher - the Guru!