Encrypting Nexus 7 vs. screen lock I read in this article that a non-encrypted tablet with only screen-lock on can have data read through USB.

Is this really true for example for a Nexus 7 tablet? I mean - if I lock the screen you can't get anywhere, even if you restart the device. Attempts to root it, or to hack it somehow when the bootloader is locked forces a data wipe and a factory reset. So is it really possible to get to the data in a screen-locked Nexus 7 which has no SD card?

The only way I can think about is that someone would somehow open the device and take the internal memory out and connected it somehow to a device which could read it. I presume that would be pretty expensive. If this really is the only way how to get to the data, it might be interesting to know, how much it may cost to do such a thing. If it is really costly, than it should be okay to just use screenlock.
Basically, as screen lock secures your data as long as the data chip stays in the phone(as long as debugging is disabled [or enabled with no trusted credentials] and the phone isn't rooted), whereas encrypting it secures it no matter what.

Old answer:
It is possible to access some data via usb, however other data isn't accessible, plug your tablet into the computer and wee what comes up under internal storage, that is accessible (EDIT: none of it if the screen is locked). Of course given enough time a hacker in possession of your tablet could access all the data (as long as it isn't encrypted) (EDIT: for example as you suggested by removing the internal storage and reading it with a different device).
If your tablet is encrypted none of the data can be accessed without your passcode (if the tablet is switched off).
If usb debugging is enabled or you are rooted it is even easier to access the data.