Words change meaning on intention.
This was an extremely high profile situation in which it was used in a derogatory fashion.
She used it in the context of same-sex marriage. She was being evasive on answering about her opinion about same-sex marriage and it is in that context she used the phrase “sexual preference”.
If you refer to sexual preference in the same area as same-sex marriage and you already are known to be against gay marriage, it is not rocket science to know the dog whistling at play there.
That nifty video of all the other people using the phrase in an ENTIRELY DIFFERENT context, were not using it as a dog whistle but she was.
Could Merriam Webster have made that change sooner. Sure they could have. But they didn’t. They made it after this event. Was this the first time making this change was ever on the list of things-to-do? I highly doubt it. But was it the straw? likely.
He didn’t use it intentionally in a high profile situation as a slur.
None of those people did.
But she did.
I’ll give an analogy. Gay.
Is gay a slur or not?
Depends who says, how and why.