Well, politics isn't about finding the one principle to rule them all. The folks/companies that find restricting access to children important, probably don't particularly care about some Linux devices being excluded. And other folks/companies find not restricting their Linux server really important, so that's a compromise that can be made.

I imagine, they landed on specifically open-source being excluded as the more abstract rule, because open-source in particular has a problem here.
Anyone can download open-source software and run it. In most cases, there is no central backend, which could handle age verification.

And even if there is, it's not really financially viable to offer that for anyone who downloads the software for free. Requiring payment to access the binaries or the source code would also effectively kill the open-source ecosystem, which is also not in the interest of most people.

Because this is temporary. FOSS OS' will be forced to comply in 10-15years max if the law stands.

Because FOSS isn't the problem. The problem the law seeks to address is profit-driven big businesses like Meta and Fortnite who don't care to provide any protections for kids or actively target them in their manipulative behaviour, hence targeting the laws at the actual problem (non-FOSS software).