Is one man giving fellatio to another man heterosexual? He said it as an insult which would mean he thinks homosexual acts are degrading. To him, at least, male on male oral sex is something a man should feel bad or ashamed of doing.
He has every right to think that, but that doesn’t change the fact it’s homophobic in nature. To give benefit of the doubt, it is a very popular American colloquialism, so he could have said it without consciously thinking about what he’s actually insinuating with that term and he later addressed his current awareness to the true intentional meaning of using “cocksucker” as an insult.
I do agree that “faggot” is a far harsher word as it is said with far more consciously aware insinuating intent. Meaning, most people know exactly what they are referring to when using that as an insult.