Many of that list are good but a few go too far IMO:
Smooth not nobbly tyres ( for the road). Tread doesn't grip more and is harder work to pedal - pump 'em up hard - see previous point.
If you're riding smooth clean tarmac, then yes, bald tyres will be fine, but a bit of a groove helps bite through the little bit of mulch and stuff I ride over, which is on most less-motorised roads here most of the year. Keep the nobblies for deeper stuff than you'll get on roads, though.
Get saddle height right - probably a good bit higher than most "beginners" think.
...and probably a good bit lower than hip-rocking sophomores think. Your weight should be shared between your saddle and your legs pushing the pedals (which is why you find the saddle harder as your legs tire).
A soft saddle is likely more uncomfortable than a hard one - though the latter does need to suit you.
Hah! No, although a soft saddle also needs to suit you and not just be a fat pillow.
Most braking should be via the front brake. Learn to brake hard safely with the front. You just can't stop quick using mainl.y the back brake. Care is however needed if turning corner or slippery surfaces
Don't brake while turning or crossing slippy stuff if you can help it: brake before and roll through.
You can't stop
as quick with the back but if you're careful not to skid, it can be pretty quick, it adapts to braking over lumpy descents better (the back naturally releases a bit if you hit a bump, whereas the front will often grab) and it's better than not stopping
Don't dismiss drop handlebars or toe clips / clip in pedals as a "racer only" thing. Both have advantags for casual cycling too - although don't suit everyone.
Advantages but drawbacks too. They're basically netural, changing the trade-offs/balancing act. Maybe you'll think they're change them in a way you like, but don't worry if you don't.
Low gears are a good thing - many / most road-oriented bikes ar egeared far to high
Use 'em if you've got 'em, but don't be afraid to get off and push if you've not got one low enough. The pros used to do it lots - see 3min in, for example:
View: https://www.youtube.com/watch?v=41lbl6q12Kw