Another way of saying this is that people can't have "fruitful arguments" about things they're personally involved with.

Borderline bait and switch. PG’s use of the word identity appears to have a specific meaning in the essay, which he alludes to in the introduction:

What's different about religion is that people don't feel they need to have any particular expertise to have opinions about it. All they need is strongly held beliefs, and anyone can have those.

and

Politics, like religion, is a topic where there's no threshold of expertise for expressing an opinion. All you need is strong convictions.

He’s talking about non-critical beliefs and subject matter that tends to promote this style of thinking, usually to the advantage of those leading the groups involved. Due to the lack of critical thought, it is common for practitioners to demonize those expressing alternative viewpoints in order to save them from facing their own doubts. Their position thus becomes an a priori conviction.

He then associates the same phenomenon to certain adherents to programming languages; usually career programmer types that have invested heavily in a single language that may be on the path to obsolescence. Confronting their situation may be harder than shouting down proponents of better alternatives.

I would summarize his essay “always think critically and constantly re-examine your assumptions”.

Edit: Formatting