I disagree because any and all words and expressions can be used to bring the dogs to the garden. Coupled with your reasoning, this would mean that all words and expressions are always dogwhistles.
Instead I think that it’s better to see a word/expression as a dogwhistle if it’s within a certain context, and that context is mostly available to a hate group but not to outsiders. This has a bunch of consequences*, but in this specific case it means that we’d need to look for further actions and words from Squabbles’ admins to know if it’s a dogwhistle or not.
Or even a lack of actions/words. If transphobic and related content becomes commonplace in their site, and they do nothing against it, I think that their very silence would be already enough to label it as a dogwhistle.
*e.g. it explains why dogwhistles are often found alongside each other, or why they keep changing, or even how they actually work on a discursive level.