Hare Krishna!
All glories to Srila Prabhupada!
I believe there is a misunderstanding here.
Scriptures have elaborately glorified and recommended being attached to the lotus feet of our Guru/ spiritual master.
For instance, we are daily singing in the Guru puja-
śrī-guru-caraṇe rati, ei se uttama-gati,
je prasāde pūre sarva āśā
Attachment to the lotus feet of Sri Guru is the perfection that fulfills all desires.
Even during mangala arati of the Lord, we are singing Gurvashtakam, glorifying and offering respects to Sri Guru!
So how is it possible to conclude that attachment to our gurujanas can negatively impact our devotional service?
In the lecture that you are referring to, Prabhuji has specifically mentioned that being attached to one’s own ideas in any relation is dangerous, including the relation of a guru and disciple. If a disciple maintains his own ideas and is attached to it, as in the case of Diti being attached to her ideas while talking to Kashyapa, then the disciple can land into trouble.
As per the lecture, the solution recommended for this problem is chastisement by guru!
This will help one get out of one’s attachment to his own ideas/ thoughts.
So to conclude, the attachment being mentioned in the lecture is the attachment to our own ideas in any relation and not the attachment to Guru.
Hope this helps.
In your service,
Ram Krishna Dasa