I'm not saying they're bad suggestions but most of us who get to the point of pinning them down have very well tried songs and toothpaste and toothbrushes. A toddler who needs pinning down isn't likely going to go out and buy a Spider-Man brush and brush his teeth with no fuss forever more. How could we not have thought of it sooner?
I think they're all perfectly good suggestions, when they're being presented as suggestions, but some of the tones of "don't pin him dow! just sing a song" as if it's never occurred to OP make me think they didn't have such stubborn toddlers.
I wrestled (no pun intended) a lot with what I should do. Pinning him is quick and although he cries at first, he smiles when we are done and applauds himself. DH picks him up and holds him in his arms, facing him forwards. DH's arms go over DS' arms, pinning them down and then I brush his teeth as quickly as I can.
I'm not willing to leave them unbrushed or let him do it himself.
I am not a "deal with it" kind of parent generally. Never sleep trained. Never been left crying for more than a few seconds. His feelings get validated constantly. Tantrums are treated with firm boundaries but patience for his big feelings. And while he's getting his teeth brushed I speak with him and sympathise and try to explain why.
When it comes to health stuff, like getting vaccinations or brushing his teeth it simply has to be done. I do not enjoy pinning him down but I won't let his teeth rot.
If your parents told you they'd pinned you down as a child because you wouldn't brush your teeth would you really be angry with them? Because I wouldn't! I would say good! I'm glad my teeth got brushed.
What about if they didn't vaccinate you because when you got to the doctors you said you didn't want to? Would you really thank them for respecting your feelings?
Surely every parent has a line where, if all else failed they would have to pin their child down to get something done. You might disagree that toothbrushing is worth that but I can't believe parents saying you should never do it.