Contrary to popular belief, it's not that simple for one to just go to a Siyum during the Nine Days and eat meat at the meal. While undoubtedly it isn't forbidden, many authorities limit those who can partake of the meat to immediate family, and 'close friends' (defined as someone who you would invite to a meal at another time), with some even limiting it to only 10 close friends.

The Siyum in question might have been kept Pareve to follow those opinions.

Source: Kitzur Shulchan Aruch, with footnotes of the Mishnah Berurah. I just read that section today (a couple days behind the daily calendar) but don't have it in front of me now. I'll try to add in the sources tomorrow.

As to why the person you asked got all defensive, well, maybe he felt he was being intruded on in his conversation. He may have been speaking in public, but that doesn't mean he was speaking publicly ;)

Contrary to popular belief, it's not that simple for one to just go to a Siyum during the Nine Days and eat meat (and wine, as it's part of the same custom) at the meal. While undoubtedly it isn't forbidden, many authorities limit those who can partake of the meat to family and 'close friends' (defined as someone who you would invite to a meal at another time, and some limiting it further to 'those who strengthened the hands of the learner'), with some even limiting it to only 10 close friends.

The Siyum in question might have been kept Pareve to follow those opinions.

Source: Kitzur Shulchan Aruch 122:8, with footnotes of the Mishnah Berurah.

As to why the person you asked got all defensive, well, maybe he felt he was being intruded on in his conversation. He may have been speaking in public, but that doesn't mean he was speaking publicly ;)