Sure, but that's not the issue here.

The issue is what kind of discrimination is allowed. In common law countries like the UK, as a rule of thumb, things are permitted unless there is a law against it. With discrimination, discrimination is permitted unless you discriminate over a protected characteristic.

You can discriminate based on qualification, for example. You wouldn't want to hire someone who wasn't qualified for the job. So discrimination is in fact widespread and usually appropriate. It's just that when we talk about discrimination we're usually talking about the bad kind, ie when people discriminate because of sex, race, disability, etc, in a manner that is not considered "fair" by reasonable people.

Sex and gender is an even more nuanced example. Usually it's wrong to discriminate over this, however in some cases it's appropriate. A women's domestic violence centre may require that it only be staffed by and only support women - particularly because the women they're supporting may have a valid fear of men, regardless of whether the specific men they're fearing pose a threat.

Anything else, that's fair game. You could refuse to hire someone who was ginger, provided you can establish that wasn't because of race or something. You could refuse to hire someone of a certain height, so long as you can establish it's not a disability. In some cases, like with sex and gender, height may even be necessary to discriminate over for what the job is.

To take the football example further: it might be appropriate for Arsenal not to hire a Man U supporter as support staff, in a similar manner to qualifications. However this happens so infrequently that it isn't worth becoming a protected characteristic. Someone refused a job at one place can still find work elsewhere; the discrimination is not widespread and has minimal harm, while preventing the discrimination may harm the effective performance of the business.

There's nothing clowny about any of this. You just have a lack of understanding of what this is, and bought into a headline designed to make you disgruntled to grab your attention and get you to engage.

if someone supporting another sports team would “disrupt the harmony of the office”, then the workers there are children that need to grow the fuck up

This is true, except that it's more of a "should" than a "need". Employers should be fair and give equal opportunity, but they only need to do so within the bounds of the law. The rest is up to them to decide - the law isn't meant to dictate everything, nor should it.