All of this is true to an extent, but I think Browny was comparing Mewtwo when he received absolutely no buffs, to now.
Most of what I said applied to that period.
Actually, let's ask this another way: What did the buffs actually do for Mewtwo? What strengths did Mewtwo not have that he does now? What weaknesses did he have that the buffs removed? Because if you actually look at it, it's probably less than you think.
The big one is mobility, yes. Going from 1/1.7/1.15 walk/run/air to 1.2/2.05/1.25 is a big boost, but 1/1.7/1.15 is still extremely respectable. He was as fast as or faster than Mario at everything except walking. The mobility buffs are a great argument for why Mewtwo is top tier now, but the lack of them wasn't crippling enough (or really at all) to warrant Mewtwo being low-tier.
Another one is shield safety, but the buffs helped M2's shield safety a lot less than you'd think. The only move that was unsafe prepatch and safe postpatch for which that actually matters is Nair.
What about combos then? As I said, landing Nair shenanigans weren't a thing, but Nair itself was still very useful in combos. It's actionable only 6 frames after its last hit and autocancels from the same frame. In fact, none of M2's aerials had their non-landing frame data altered at all save for taking 1 frame of startup off fair. This means his aerial combo game was exactly the same as it is now (maybe slightly worse due to the lower airspeed), and he could start combos in all the same ways as well, save for, obviously, landing Nair.
So his combo game was largely the same, his mobility, while worse, was still quite good, and his shield safety wasn't much different. His disadvantage wasn't really buffed, his advantage was basically the same as well, his tail was always disjointed, his perfect pivot was always dumb, and he always had Shadow Ball. The only big things he gained were the mobility buff he didn't need and improvements to Nair.
I'm not trying to undersell either change, both did quite a bit for Mewtwo, but other characters who got similar buffs moved nowhere near as much. Additionally, Mewtwo's tier jump happened independently of the buffs, which proves that how good he actually was wasn't the deciding factor in his placement, and misinformation about Mewtwo has been everywhere since launch. Considering all that, I think it's pretty safe to assume that Mewtwo was highly underrated at launch. He wasn't top tier, that mobility buff was big and prepatch top tiers were much stronger than current top tiers, but he wasn't nearly as bad as people made him out to be.
One other thing:
This understates the mobility buff quite a bit- he went from being slower than Donkey Kong to being faster than Sheik. Those are huge mobility buffs in a game where mobility is king.
Fun fact: Every single E-tier save Ike has either top 15 run speed, top 15 airspeed, or both, along with a walk speed above the median. Meanwhile, every single B-tier save Falcon and Lucina has either a runspeed or airspeed in the lower half of the cast. Mobility can make or break a character, but it's not even close to king, in this game or in Melee. It's more like a big political party.