What a character is supposed to be is an entirely different thing from what a character actually is in a story. Which is an important distinction to make, because delivery of a character is everything. Technicalities about what powers a character has and how they can use them and how skilled they are with them and how many flashing lights those powers can create don't mean anything when they're separated from a character, in action, in their story. You can (I mean, within reason, I suppose. There're some things that just... can't be done well,) use the bio of a Mary Sue to deliver a good character in a story, and you can use the bio of a well-written character to deliver a Mary Sue. You got to look at things like how the techniques affect or reflect their character, or things about the reason for the character having said technique in the first place.

A Mary Sue has lots of techniques and powers essentially for bragging rights - you'll see a million Sues get bestowed heaps of abilities and powers which will get used once to show off and be forgotten, which aren't shown at all, or which are used but don't even affect the plot or anything. They're supposed to be all skilled and have all these techniques and all, and in practice they just look like a sockpuppet being held up by a badficcer who's surreptitiously saying 'Lookit me, I'm real cool!'
A well-written character who has lots of techniques and powers will actually have those techniques and powers relevant to them, whether it's as extensions of their character and backstory and all (be a bit weird if a ninja character couldn't be sneaky or any such,) or whether those techniques and powers will actually contribute to the story at hand. It can go a lot of ways, from the ninjafellow using his sneaking to bypass some obstacle, or maybe he gets all angsty about his sneaking because his love interest has an unnecessary hatred towards sneaky things. However it's done, a well-written character's skills are directly relevant to them and their plot, and if you took away said well-written character's skills, they would be losing important aspects of their character. If you took away a Mary Sue's skills, they would be losing annoying bragging rights. And of course, the Mary Sue and the ninjafellow could very easily have the exact same skills and techniques and bio - it all comes down to how it's delivered in the story.

There is a bit of a problem with large numbers of techniques generally being associated with Mary Sue types. It, once more, goes down to the issue of how the techniques relate to the character and why they're there. An interesting character generally doesn't need that many skills or techniques to be interesting. Sometimes it's even better to have less, to put emphasis on the ones they already got. Meanwhile, a Sue, which has no interest in any of that kind of stuff, can stuff themselves with as few or as many techniques as they want, because the techniques, for all purposes, essentially don't exist.

So, lots of techniques does not equal Sue. It might imply a Sue, it might be a symptom of a Sue, but it doesn't equal a Sue. Don't get me wrong - I will be very suspicious of a character with heaps of skills as opposed to one with a more moderate amount. And I do not think that I am incorrect in being so. But, nah, doesn't outright indicate one, I don't think.