Oh yes, you definitely have to pull the mulch back to sow seeds! The idea with the deep mulch is to make it so deep a layer that weed seeds don't germinate because they can't get enough light. So using the same logic, our veggie seeds won't germinate without light either. What I did was to build the layer of mulch over a period of months, adding to it whenever I had organic materials. Consequently it is an irregular layer as here it is straw, there, hay, there chipped tomato plants etc. By the start of this spring, the layer was about 6" deep over the entire garden. Then, when I want to plant, I pull back the mulch just in the spot I want to plant. I mean, really just a small hole in it, enough that light will be able to get through. I plant my seed but leave the mulch pulled back until the seed germinates. Once it becomes a seedling, the mulch is piled back around its stem.I've done no-till, deep mulch method for several years - using hay, chopped up leaves, grass clippings, etc - they worked to some extent, but didn't keep the soil moist underneath.
Perhaps I didn't put it on thick enough, and I certainly went through many years of a hard time direct-sowing small seeds, due to the mulch.
And if I pulled back the mulch to sow seeds, then the weeds would also germinate!
I've found that it works quite well as a weed deterrent. The nature of gardening means I'll never get out of weeding altogether, but compared to when I started the garden, my weeding efforts are quite minimal these days. I'm constantly reminded of the quote from Jurassic Park - "Life will find a way". Weed seeds will still try to germinate even though all that mulch. In fact, sometimes the mulch itself contains the seeds - for example, grass seeds from within the hay. But they're quite easy to see and pull and I'm finding that about 20 minutes every other day allows me to pull all the weeds in a garden area that is about 50' long and 16' wide.
I find that the soil underneath stays really moist. I did have the mulch out there this winter so any snow we got (not much) melted through it, but then we had an extremely dry March and April. I expected the moisture content to be low under the mulch but when I started to plant my seeds I was surprised to find how moist it was. This month we've had about 10" total rain so far and we're finding the mulch is keeping it almost too moist now. The same action that holds the moisture IN, preventing it evaporating on the hot summer days, is also keeping the moisture in when we have it in abundance and the poor little roots are a bit water-logged. Most of my garden is still at the tender seedling stage because with too much rain and not enough sun, they haven't had the conditions they would like to really take off and start growing. I'm ready for the weather to warm up and dry out just a little

A tip for you if you are looking for things to use your chipper on. Last year I grew a LOT of tomatoes. At the end of the season, I pulled the plants and piled them up outside my veggie garden fence. I let my goats out to free-range sometimes in the evenings and they found the pile. I was a bit concerned about them eating it since the tomato plant is in the nightshade family and I worried it was toxic. But they ate it in huge quantities and never got sick so I guess its okay for them. My doe was pregnant at the time and her kids arrived healthy too! Anyway, I turned the pile for them a couple of times so they could get all the foliage and eventually the pile was stripped to just the stems. I left them to dry out for several weeks and then ran them through my leaf shredder, which also does small sticks. It chipped them and then I used the chips as mulch in the garden area. That has been the BEST mulch so far. No weeds have tried to grow through it at all. I wasn't sure about its qualities as mulch around new plants, so I laid it on one of my walkways between beds, just in case, but I'm really impressed with it. At least I know what I'll be doing with tomato plants in the future!