If someone proclaimed themselves an "open source dev" but didn't know linux to at least some extent, my inside voice would dub them heretical and proclaim yet another fool that doesn't even know what they're saying.  I would probably be compelled to at least ask sarcastically just exactly what open source they work with then, as microsoft or mac anything certainly isn't.  If something *is* developed as open on windoze, it's likely cross-platform by nature, probably works worst under microsoft trying to make a square peg fit a round hole, and the person handing the windoze port hates their life.

As a "non-developer" infrastructure geek, working with a lot of devs over the years I tend to throw developers into around 4 different buckets, mostly windows, linux, application, or hardware devs. 

1) Windows devs tend to be all about the MS Visual * IDE's for things microsoft, or some other framework built around microsoft-y languages. They usually went through some college or tech school that was funded on grants by Microsoft kickbacks, started at mastering Excel, and moved to Visual Basic, C++ or just blob together stuff in a .net hybrid framework.  Probably don't even know how to maintain, network, or secure even the windows server they work atop, and pretty narrow in vision around their given choice of dev languages or frameworks.

2) Linux devs are usually versatile hacker types, jack of all trades, masters (eventually) of a few, probably never bothered with much "higher education", but know their way around not only their code but also the systems they run on.  They usually know networking well enough to tcpdump things or setup bonded interfaces, know at least basic security, and know enough to set up their own web servers, runtime environments, or whatever they work in most.  They probably know more about windows than windows devs do, and tell no one lest someone makes them do that too.  They're either hardcore and use a linux desktop, otherwise probably a mac, or hate their lives stuck using a corp windoze laptop and living through an ssh client.

3) Application devs tend to be abstracted from any OS, usually more involved writing glue between services and frameworks more on the backend, or more focused on front-end UI for users glued to backend services.  These folks are usually not concerned with the OS or systems they run on, so long as the OS has a means of running their code directly, or some client ala web browser or dedicated front-end to access it.  They just need a place to drop code that works to do what they're creating, only know enough server/OS to triage their runtime environment basically, or they go find someone to fix their environment whatever OS it's on.  I usually lump database folks in here too generically, as some like this and other applications/services can be massively complex in their own right that an OS is the least of their concerns - Oracle and SAP folks are good examples of this.

4) Hardware devs tend to be interesting folks, like baby pigeons, they're often heard but never seen.  I occasionally meet them working with infrastructure hardware vendors, and usually pretty interesting folks, but all kinds of different mad scientists.  I find them more like applications folks that just need access to whatever OS their development environment works best on, be that windows, mac, or linux and only know enough to support their use of the tools.  More and more this changes still as almost any hardware including networking, storage, embedded device systems, or IOT gadget runs linux today, so even in hardware development there is no escaping it fully.

Is it fair to expect cross-platform skill or expertise?  No, but these days it can only help as orgs move toward "cloud", and the cloud is probably 80% linux from the network hardware to the hypervisors that run your little windoze system on.  NOT knowing ANY linux is merely sticking your head in the ground hoping it will go away, until it eats you.  Hell, even Microsoft Azure's best selling product is linux, so it's not hard to understand why the Beast of Redmond is finally playing nice(er) with linux.  It's a hard pill to swallow for those that have only ever known working on windoze, those that won't evolve outside their bubble will probably die in their careers doing so as with still 80% of desktops still using windows, Active Directory, and all the supporting systems to keep it safe and secure, someone has to support those stuck using it.

-mb