As a veteran of dozens of marathons over the years, ranging from 2:40 to 4:20 (most recently 3:07 at age 58), I think the plan is reasonable IF you have the genetic talent.
Your previous times speak to your talent, especially if your training mileage was low. Still, if this plan is MUCH less mileage than what you were running for the 3:09, then it will be very tough to go sub-3. Well, it will be very difficult to actually run the prescribed pace/distances. If you can, then, yeah 2:59 would be doable!
Generally, I don't like training at marathon pace, and I think there would be more benefit to making the "Day 1" workouts a little faster and shorter. That is, a relaxed warmup of 2 or 3 miles, then 4 to 5 miles at maybe 10-mile race pace, then a few easy miles. On the other hand, the author calls "6:40" MP pace, so that's already a lot faster than 6:50 (what I would call 3-hour marathon pace).
I'll add that the plan's Week 10 "Day 1" calls for TEN MILES at 6:40 pace, which is a heckuva run! The last time I went sub-3 (2:56 in my mid-40s), there was NO WAY I could run that pace for that long on a solo training run.
But I'll also add that at super low mileage, training paces get kinda mixed up. Actual race-day marathon pace would be slower than when "fully trained," plus with low mileage, you're better rested for each run. At age 57, I adopted a similar plan as this one (three running days a week, never consecutive days), and could run 15 miles at marathon pace (but that was only 7:52/mile -- race day I ran 3:26:23).
For "Day 2" I like the 800m repeats at 5K pace. But, wow, I've never managed more than 8 of them (usually only 6). The plan maxes at 10. Still, it targets your VO2max, helps your running form, and makes marathon pace seem "slow." My best marathons were always when I was equally trained for 5K racing.
The "Day 3" long runs are the key. Again, I never target MP during training. And I never worry about pace during long runs, just distance (18-23 miles), but if you can follow the plan's inclusion of MP pace, that will certainly boost your confidence. One thing I would change, though, is to progress the distance on ALTERNATE weeks. Like, one week 18 miles, the next 16, the next 20, back down to 18, or the like. Three or four weeks from Marathon Day, I include a 23-miler, even if I have to walk some near the end.
I've also tried one-day-a-week training: only a single long run and six days off. I never broke 3 hours with THAT plan. Maybe 3:40 or so back in my 20s, but the plan was only 8 weeks long (7 runs total). Ha! Good luck.