No, they really mean it. Even if you write it in double bold and highlight it in orange and make everybody signing read it out loud first, you can't enforce terms that aren't legal.
For example, in the US I believe it's pretty common for renters to be obliged to pay for or fix up normal dilapidation of the property when they leave. But in the UK it's illegal to require that for residential property. If you rent out houses in the UK, you're going to be paying to repaint them every few years, replace worn carpets, and so on, because the law says the tenant doesn't pay, and if you add a clause saying "Actually the tenant agrees to pay anyway" the law says your lease acts exactly as if you didn't add that clause except now you also lose the right to get courts to enforce your lease because it was illegal and you lose certain other protections because clearly you're an abusive landlord. So bad luck you made things harder on yourself.
While there can be unfair terms, which are then unenforceable, if a tenant and landlord explicitly and freely agree that the tenant will pay for something or do something this isn't necessarily invalid as this can be interpreted as being consideration (i.e. what the tenant agrees to do in exchange to be granted the lease). Certainly there is no loss of "right to get courts to enforce your lease" or "other protections".
For example, in the US I believe it's pretty common for renters to be obliged to pay for or fix up normal dilapidation of the property when they leave. But in the UK it's illegal to require that for residential property. If you rent out houses in the UK, you're going to be paying to repaint them every few years, replace worn carpets, and so on, because the law says the tenant doesn't pay, and if you add a clause saying "Actually the tenant agrees to pay anyway" the law says your lease acts exactly as if you didn't add that clause except now you also lose the right to get courts to enforce your lease because it was illegal and you lose certain other protections because clearly you're an abusive landlord. So bad luck you made things harder on yourself.