I disagree with that statement completely. We are knowledgeable enough in all platforms and can easily navigate from one to the other, but you can't call yourself an expert in everything. It simply isn't feasible due to time constraints.
I never called myself an expert in everything, nor would I consider myself one. I'd consider myself an expert in a few specific areas, and a "skilled engineer" in a wide range of areas. Your statement that "skilled engineers spend entire careers on narrowly focused aspects of one single PLC platform" is blatantly untrue not just for me, but for the vast majority of those who frequent this forum, based on the extensive and broad range of knowledge I see every day. Ken Roach can give someone intricate detail on a 30-year-old Allen-Bradley PLC one minute, and then turn around and help me interpret a wireshark trace on a Control Logix the next, and then head off to help someone out with a Red Lion application, all before his morning coffee. Geospark will sit down and pen a 2,000 word post on just about any Allen-Bradley related topic you can think of, with enough technotes and cross references to keep you busy for hours. geniusintraining can help you out with parts and solutions for just about any combination of automation equipment you can throw at him. Phil Buchanan runs training courses on Allen-Bradley, Siemens, Omron, Wonderware and Mitsubishi, and has at least passing knowledge on many other brands besides. I'm sure that none of these people would consider themselves "experts in everything", but if we are to agree with your statement, then none of them are skilled engineers either.
To elaborate some more:
I certainly think that as an engineer you should learn as much as possible & know a little bit of everything. However, as a manager or someone who's making a purchase of a piece of equipment, you need to standardize across a single site. There's no reason to have your team learn 6 platforms.
Really?
No reason? At all?
You're right that if you're in the position of ordering equipment for a single site, you should standardise as much as you possibly can. But that's not the way things go, and so engineers need to be adaptable.
I personally can program PLC's and HMI's from Allen-Bradley, Siemens, Modicon, Schneider, Omron, Red Lion, Delta, Panasonic, Eaton, Automation Direct, Pilz, Mitsubishi, IFM, Sick, GE, Unitronics, ranging from the MS-DOS based Siemens S5 through to the most recent version of AB Logix Designer. I can develop SCADA systems in FTView, Wonderware, Citect, WinCC and Ignition. Sure, I'm not equally good at all of them, but I can at the very least get by in them all. And you know what? At my current employer, I have been at some point paid to work on every single one of those brands by our customers. If your employer is happy to tell customers "sorry, we don't have anyone that can help you with that, find someone else", then sure, I guess your employer has no reason to have their team learn 6 platforms. But if you're going to turn to my boss and suggest that my knowledge of more than one PLC brand is of no use to him, he'll laugh in your face.
Sure, there are roles where you only need one very narrow, but very in depth skill set. I'm sure that Tesla employs engineers who know more about battery technology than you or I could ever hope to know, but wouldn't know a VSD if it fell on their head. I'm sure that there are OEMs who develop a very specific type of machine using a very specific type of platform and have engineers who know that specific platform inside and out, but have very little experience outside it. But my observation would be, that's the exception, not the rule. Even within a factory that is exclusively, say, AB, an engineer would be very little use to the company if all they knew was servo motion. They'd most likely be expected to be highly competent across PLC programming on RSLogix 5, RSLogix 500, RSLogix 5000, and in ladder, FBD and ST. They'd be expected to be competent with FTView and PanelView Plus applications. If the site used a different SCADA platform, they'd be expected to be competent across that as well. They'd need to know VFD's, networking (ethernet, devicenet, controlnet, RIO, DH+...), safety systems, and sensors. And that, again, is a site with just AB. The reality is, there are precious few sites that have managed to keep the bean counters in line well enough to have the luxury of only having one brand to deal with. For the rest - including the site the OP is working at - the more areas you're "skilled" in, the more valuable you are as an employee. And if you work for a small OEM like me, well, you simply do not have the luxury of specialising in one aspect of one platform and ignoring the rest of the field.