From a male perspective, I can support the notion that death grip is a bit of a fallacy, having masturbated almost daily for most of my adult life when both single or in LTRs.
There have been many times over the years when I've not been able to cum (and 99% of the time when I can) and it's either been circumstantial by not being able to relax due to some kind of stress, worry or anxiety, or more often than not, I'm simply just not turned on enough.
From my perspective, the orgasm process starts in the brain and is not just the result of sufficient manual stimulation. For example, I remember as a young teen when I got my hands on my first porn magazine (long before internet etc) and literally ran all the way home to look at it. I was so utterly excited and turned on by the anticipation and what I was looking at, that within about 5 seconds of touching myself it was all over. I was already so massively turned on and excited, the manual stimulation part was just the final mechanism.
Where as if I'm not excited, or there's been no chemistry built up, or there's a little bit of resentment over something from earlier in the day or some other thing that's bothering me, I'm just not far enough down the orgasm road to complete the journey and no amount of stimulation only will get me there if my head hasn't unlocked the gate.
Also, the moment you become even remotely conscious of it taking a while, for me at least, it's game over and there's no coming back (no pun intended) as the additional self consciousness and anxiety makes the problem worse.
I guess what I'm trying to say is that in many (not all) cases, orgasm issues will likely be in the head, not the result of excessive masturbation, with or without a tight grip.