Is it true that in the first 30 seconds an interviewer makes up their mind about a candidate?

Recently a candidate's experience of attending an employment recruitment day with a large international airline was shared with me. Prior to the day candidates were sent information about what to expect so that they could prepare. The day involved a mixture of practical assessments and a panel interview.

The 6 candidates arrived on the day. Candidates had a different backgrounds and experience and also varied in age. Their physical presentation was different. 4 of the candidates wore suits, the fifth tailored trousers and a jacket and tie. The sixth wore tailored jeans and casual jacket. As the day wore on candidates became more comfortable talking with each other and of course the discussion reverted to the employment assessments completed and the company. The candidate wearing tailored jeans asked his colleagues should he have worn a suit. There was a little quiet before one of the candiates rescued the pause and said it didn't really matter as long as he was technically sound and articulate in the interview it really wouldn't make any difference. It was clear from the uncomfortable pause the other candidates thought dressing more formally was appropriate for the situation.

I wonder whether the panel interviewers had the same reaction as the candidates. With a prestigious international airline, reknown for its immaculate presentation of its employeees, was there an underwritten expectation to dress formally for the interview? What do you think?