We learn in Bava Metzia that one is allowed to lie if someone asks him if he has learned a certain masechta. For example if someone walks up to me and asks me if I’ve learned maseches shabbos(and he’s not asking for a halachic question see tos.), even if I have, I’m allowed to reply “no, I haven’t”. This is to avoid pride and haughtiness. If so would it be proper to avoid making siyumim?
Not only is it not wrong, it is a mitzvah. The Rema writes in Shulchan Aruch
הַגָּה: כְּשֶׁמְּסַיֵּם מַסֶּכֶת, מִצְוָה לִשְׂמֹחַ וְלַעֲשׂוֹת סְעֻדָּה, וְנִקְרֵאת סְעֻדַּת מִצְוָה
When one finishes a Masechta, it is a mitzvah to rejoice and make a meal, and it is considered a "seudas mitzvah".
Perhaps, I don’t have a source on this, just my own hunch, there’s a difference who is asking the question. It’s not inherently haughty to say you did a mitzvah of learning. However, some people who are not familiar with that, will look at it as being haughty.
In the Gemara BM, it’s talking about a random person who is not learned, and by answering him in the positive, it will seem to HIM that it’s haughtiness, so they change the answer in an act of humility.
By a siyum, the Gemara says Abaye made a seuda for the other “rabbanan” meaning people who understand what learning is about and that it has nothing to do with haughtiness, and only to do with service of Hashem and mitzvohs.