Is air sealing really necessary before adding attic insulation?

Your neighbor is right, and this is one of those cases where the boring prep work matters more than the fun part. I did this exact thing in my own attic a few years back, sealed everything first, then blew in a bunch of cellulose. The drafts in the bedrooms below basically went away that winter, way more than I expected from just adding insulation on top. Here's why the little holes matter so much. Warm air in your house wants to rise, and it pushes up and out through every gap it can find in the attic, like around the chimney chase, wiring holes, the top plates where the walls meet the attic floor. As that air escapes, it pulls cold outside air in through leaks down at the bottom of the house. That's the stack effect, and it runs constantly all winter. Insulation slows heat moving through it, but it does almost nothing to stop moving air. That's why piling fluffy stuff over a hole is like putting a sweater over a screen door.

The footprint of missing insulation in the corner matters too, so fill that in, but the penetrations are what drive the airflow. And they interact, because air blowing through and around loose insulation degrades how well the insulation even works.

As for DIY, it's very doable, just slow and unpleasant. The trick is doing it methodically instead of hunting randomly. Go up on a cold day and look for dark staining on the existing insulation, that's dust from air moving through, and it marks every leak for you. Foam board and caulk for bigger gaps around the chase, fire-rated caulk or the stuff made for sealing around anything near the chimney, and spray foam for small holes and wiring penetrations. Give yourself a couple of sessions up there because you'll miss stuff and need to go back. One warning though, keep the clearance off the chimney flue whatever you do, that's the one spot where the wrong material against it is a real fire risk. Then blow your insulation after everything's sealed and you'll get way more out of it.

Wish I'd known about the attic hatch sooner, that thing was the worst leak in my whole house and I walked right past it every time I went up there. I built a simple cover out of a piece of rigid foam and stuck weatherstripping tape around the edges, and the room under the hatch stopped feeling like a freezer overnight. Also second the cold day tip, I used a stick of incense and watched the smoke get sucked sideways near the top plates, made finding holes way easier than guessing. Once I did that, the rest of the sealing went fast.

Went the pay-someone route on mine and honestly didn't regret it. We're both bad with ladders and our attic's a crawl, so a local crew came out, spent half a day sealing, then blew everything in same trip. Cost more than renting the machine for sure, but my knees thanked me. One thing they found that I never would've: a couple of old recessed lights down in the ceiling below were basicly wide open to the attic. Air was pouring through them. If your place is an older farmhouse there's a decent chance you've got some of those hiding up there too, so check before you start. Either way, don't skip the sealing. It's the whole ballgame.