Graphics performance on MacBook Pro 15 / 2015
First, can I trust this source?  Any way to cross-check?


Everymac.com should be considered a trusted source.  This can be cross checked by looking at two sources:


Apple website; tech specs of 2015 MBP will tell you it uses an Intel Iris integrated graphics chipset
Info about Integrated video on Intel based Mac computers; also on Apple's website.


Basically, Intel Pro Graphics (integrated graphics) uses shared memory of up to 1.5GB of system RAM.  There's no dedicated GPU.


  Second, is the dedicated graphics card required if I only need to run
  SolidWorks occasionally?


Do you need it?  No, it's not required.  You may find that the performance (lack of) is something you can live with if you are only using it occasionally.


  I don't need so much SSD space, and wouldn't like paying for
  high-performance graphics unless I really need it.


One factor you need to be cognizant of is that these models are completely un-upgradeable.  The best advice that I can give (which I personally live by) is purchase for what you think you will need tomorrow not what you need today.  Further, if the performance for what you want to do today is marginal at best, it's not going to get any better in the future.

Keep in mind that the phrase, "Gee, my graphics is just too responsive" and "I wish I didn't have so much storage" was uttered by no one, ever.