Too bad, because that's not a valid argument.
Well, looking at it from the standpoint of a japanese-speaking person, his argument is, in fact, valid. The "dana" gobi is used when one's expressing acknowledgment. If someone were to use this to comment on oneself, it'd sound like that guy is a complete jerk and simply incorrect. I can remind of two occasions when Luffy was commenting on Hody and using it. I hope these two examples will give you an idea of what it means:
頑丈だなあいつ・・・ - That guy sure is sturdy…
タフさは億超えだな・・・ - His toughness sure does exceed (that of) a hundred million...
You can say in similar fashion that a building is really tall without mentioning the word 'building' at all and it'd be natural to assume you're talking about the building rather than yourself, right? Also, please take Mangastream's translations with a grain of salt and take a note that I was literal for reason. All comes down to the fact that Mangastream's translator got it all wrong (not the first time, not the last).