In the modern day, you can't see tendons through typical firearms handling gloves [that I've seen] but that doesn't mean they wouldn't in the future. That said, I don't think I'd notice. I've honestly never noticed being able to see tendons on my guardsmen before. I don't look that close, and I do paint the hands in flesh tones, so maybe I've just never actively noticed.
I wouldn't worry about it. Painting Guard *Armies* is about quantity over quality. Spend time on the characters and upgrades if you feel you must, but spend as little time as you can on dudes with Lasguns. Honestly, crank those bastards out as fast as you humanly can, with as low a quality as you can stand. They're literally extra wounds for your special weapons and space fillers on the board. You pick them up by the literal handful when they're dying.
Resist the urge to make each one a snowflake. They're cookie dough. Roll 'em out, stamp 'em out', and bake 'em till they're done. A chocolate chip cookie tastes good whether it's perfectly formed or a misshapen mess. You've got to keep that in mind with Guardsmen. If you aren't embarrassed, that's good enough for a lasgunner.