Half of those fights he listed actually have adds on them lol.
I don't see how mastering add mechanics is any different than mastering boss mechanics. Either way you have to master...mechanics?
I won't defend Blizzard on every encounter they have made, but not every encounter (even today) is the same. You have a decent mixture of adds and those that just present you with a boss.
This isn't even a new phenomenon either, it's not like past encounters had less add centric encounters as opposed to today. Simply put encounters as a whole, both single boss and those with adds have way more going on.
MC/Onyxia - Everyone of those encounters except Magmadar, Geddon, and Shazzrah had adds.
Blackwing Lair - Almost every single encounter was just a boss, except for the first and the last one. I wouldn't call the drakes 'engaging' bosses, sans Firemaw.
AQ40 - Skeram (depends how you define it) and Huhuran are the only encounters in here without adds.
Naxxaramas - 15 bosses. Patchwerk, Grobbulus, Loatheb, and Sapphiron were the only ones without adds.
ZG/AQ20 - Littered with add bosses, only a couple in AQ20 didn't have them.
TBC
Karazhan - Too many to list, but pretty even mix.
Gruul's Lair - No add bosses, as I assume people don't consider council bosses adds. Although you did have to deal with the warlock ogres pet!
Magethridon's Lair - First phase was entirely adds.
Tempest Keep - Every single boss in here except the Void Reaver boss had adds, and a lot of them.
SSC - All bosses except Leotheras (although there was an add component) and the Council boss (hunter pets) had adds.
Hyjal - Lots of single target bosses here, although honestly the instance itself was one giant trash pull.
Black Temple - Lots of single target bosses here, except for Illidan, Shade of Akama and Teron Gorefiend (depending if you count the actually hard mechanic as adds)
Sunwell - Kalecgos, Brutallus and Twin Eredar were single target, the other 3 were add fights.
I won't go through the rest as it pretty much illustrates my point. Yes there have been solely 'boss only' encounters, but it's not like there was a time and period in this game were it was only a single boss to worry about. Through the games history, even early history there has been adds present in one shape or another and that hasn't changed as of recently.
I think a lot of it's a whine to be honest. Regardless of whether or not adds play a part in an encounter, I think a lot of people just want fairly straight forward bosses with very little mechanics. Most encounters, even in Vanilla and TBC have heavily favored adds, and they still favor add encounters to this day. It's never changed. Most of the memorable encounters feature adds for a variety of a reasons, including making encounters more difficult. Most of the time we are storming a giant fortress, it doesn't make sense gameplay or lore (lolore) wise for it to be a boss to be by themselves and not have legions at their command.