I would personally say that I disagree with Master Hsuan on that.
He does not speak for Guardian beings or Devas or other such beings. They are sentient beings in their own right, and can decide for themselves who they will or will not protect, and that is up to them.
I've run across guardian beings before, and I've never run across one that would refuse to protect someone who was worthy of protection simply because they had eaten garlic for their last meal.
The prohibition against the five pungent roots comes from the 48 Less Grave Precepts in the Sutra of Bramha's Net.
But nowhere in that, does the Buddha say that guardian beings will refuse to help someone just because they've eaten them, and in the Shurangama Sutra, the Buddha clearly explains that it has to do with the smell, and bad breath and body odor.
The monks at Shasta Abbey, avoid strong wild onions, (and the other four pungent roots) but they do eat the milder, large commercial onions in onion soup and such things occasionally, and they have a guardian spirit who watches their temple, as do many monks at many temples.
I remember asking the monks about this, and they pointed out that the purpose of this Less Grave Precept, was two parts: first, back in the day, people didn't have showers, plumbing, and the ability to bathe and wash clothes on a daily basis. Nor did they have toothbrushes and modern oral care. This meant monks would often have to bathe irregularly, and didn't brush their teeth, and so a lot of this prohibition was very practical:
If you can imagine, a monastery in the middle of summer, filled with mostly men, and they all are sweating because it's hot, and they all smell like body odor and garlic, and their breath stinks...
Yeah, that would be kind of miserable, and also, you wouldn't want to present to laypeople that way, because you'd want to give a respectable presentation of Buddhism. When giving voice to the Dharma, you wouldn't want people cringing away because of the atrocious smell of your body and breath. Like a dirty hippie!
The second reason, is because those roots function as stimulants. And because of that, they can have a mild aphrodisiac effect, that was (and is) unhelpful to people trying to be celibate. Since they are also trying to calm down their passion in general, they refrain from such foods.
The monks at Shasta, for example, refrain from garlic, leeks, scallions, ginger, and the strong wild onions.
Nowadays, we have much milder onions cultivated for general use, but back in the day, people simply gathered onions in the wild, and if you've ever had a wild onion, they are incredibly potent. Also, as the translator's note points out in the Sutra of Brahma's Net, that in the original texts, the texts themselves referred to highly specific plants, not general varieties.
Here's the actual Sutra:
FOURTH LESS GRAVE PRECEPT
On using the five pungent roots.
"Disciples of the Buddha, you should not eat of
the five types of pungent roots.* Pray, do not use any
of these roots in your food. Should you eat of
these roots willingly and knowingly, you are thereby
defiling yourself by acting contrary to this less grave
* The Chinese text names five highly specific types
of pungent roots, apparently considered by Kūmarajīva as
the equivalents of specific plants found in India. Japanese
Buddhists have traditionally identified them in more generic
terms as garlic, leeks, large onions, scallions, and ginger (or
horseradish), even though the text names only specific types.
Explanations for their interdiction include their giving an
offensive odor to the breath and their 'heating the blood'
(that is, having some aphrodesiacal effect).
Nearman, H., Jiyu-Kennett, P. T., & MacPhillamy, D. (1998). Buddhist writings on meditation and daily practice: The serene reflection meditation tradition : including the complete scripture of Brahma's net. Mt. Shasta, Calif: Shasta Abbey Press.
"Life is full of suffering. AND Life is full of the Eternal
IT IS OUR CHOICE
We can stand in our shadow, and wallow in the darkness,
We can turn around.
It is OUR choice." -Rev. Basil Singer
" ...out of fear, even the good harm one another. " -Rev. Dazui MacPhillamy