I agree. It's much easier to divide hostas in the spring before they leaf out fully. If you do it later, you are likely to damage the foliage and the water requirements are more critical when it's warmer too. But through experience I've found that hostas are not created equal in how easy they are to divide. Some will literally fall apart. Others can be teased apart with a bit of work. Others literally need a cut through the crown. For me, in my Zone 5a garden, I've found that those that need to be cut seem to divide more easily in the autumn. YMMV.
Another question is to ask yourself why you are dividing. Do you want more plants? Or are they simply outgrowing the space available? In the latter circumstance, I've sometimes just taken a clean, sanitized spade and cut a bit off the edge of the plant. Sometimes you get viable sections you can pot up. But you will likely destroy a few eyes in the process.
And please note that hostas (plural) does not need an apostrophe.
