I've certainly heard that before, and early on.
3-4 deadly with a short rest after each encounter might be comparable in encounter difficulty calculations, but it won't just be as many rounds as 6-8 encounter with the same pressures on resource management. They're going to be higher stakes, putting more emphasis on the early rounds, because if you don't eliminate enemies fast you'll be overwhelmed.
....but, 6-8 is already a range, and substituting 1 deadly for 2 of those 6-8 expands it all the way out to 3-8 encounters, and maybe some more issues with when to short rest....
I've found pretty consistently that a Deadly encounter runs about 5-6 rounds, a Hard 3 rounds, and a Medium 2 rounds. Some of that depends on the nova level and the specific monster and PC class combination, and dice luck, but the overall average is spot on. It's certainly close enough to the sweet spot described in the OP. In terms of resource management, Deadlies are also more likely to cause casters to lose concentration on spells, forcing them to use additional slots that they might not use in a medium or hard encounter.
Magic Items, being a pure boon outside of the expected math, will reduce the rounds (as they ultimately make the party work at an effective level higher than their actual level). However, martials especially benefit from having magic items, since many are active every single round of combat (offensively and defensively) and the ones that aren't provide additional options for the martial, whereas for casters they only provide additional options (unless they have a magic item that gives them access to spells above their current level).
Not following Random treasure generation rules can amplify the issue with magic items, since you are both more likely to give casters more than they would get from the random tables (which lean heavily towards martial support by percentage) and can provide precise boons to enhance casters far more often than the random tables assume (looking at all those Rods of the Pact Keeper out there, even though there is only a 2% chance to get one on table F).
And 3 deadlies isn't just comparable in encounter difficulty calculations, it's almost exact.
1st Level - Deadly encounter - 100xp, Adventuring Day 300 XP (3 Deadlies)
5th Level - Deadly encounter - 1,100xp, Adventuring Day 3,500XP (3 Deadlies goes 200xp under)
11th Level - Deadly encounter - 3,600xp, Adventuring Day 10,500 (3 Deadlies goes 300xp over)
Short rest between each encounter and you end up with 2 short rests per day.
