I'm specifically interested in whether they can legally enforce this in order to license their product
Yes, they can legally enforce this.
Software 'licences' have two kinds of teeth: as licences (for copyright mostly, but also patents etc), and as contracts.
Violating a condition of your copyright licence means you no longer have permission to use the copyright in the software, which means that your use of the software (which inevitably involves making copies of the software) is a violation of copyright, which results in you having to pay damages (money) to the copyright owner. Copyright damages may be 'statutory damages', which works a lot like a fine, or actual damages, which is based on the injury or cost inflicted on the copyright owner by your violation.
Violating a contract (i.e. failing to do what you promised to do under the bargain) results in you having to pay damages (money) to the other party to the contract. There are no 'statutory damages' for breach of contract; you just compensate the other party for whatever injury (cost) they have suffered.
The next question is whether the contract is void under some special rule.
Generally speaking, you can contract for whatever silly or frivolous promises you like (e.g. 'I promise to only wear X brand of shoes while in public'). This is called 'freedom of contract' and is given great weight by common law courts.
A contract can be avoided if it is an unreasonable restraint of trade. The exemplar of this category is an agreement by a baker to never bake another loaf of bread again. A court will not enforce that. However, there are limits on the principle. For example, a court will enforce a promise by a baker to not open a competing bakery within ten kilometres in the next two years; such a promise is reasonable (in the common law's book) as, for example, part of an agreement for the sale of a bakery business.
At most, the contract in question could be said to restrain your freedom to use whatever software you like. This is unlikely to be enough to get the contract avoided. Even if you got 'prima facie' into a special rule, the copyright owner could probably persuade the court that you fall outside of it because the contract served a legitimate purpose such as (a) protecting them from support demands or reputational risk if you use unsupported third-party software and complain when one breaks the other and/or (b) providing what is essentially a 'package deal' for the sale of the two pieces of software together.