It is so easy to record things now, and people must know that not everyone can watch a programme when it is broadcast.
It wouldn't have been too hard for the OP to have left the spoiler out of the thread title, so that people who hadn't watched it yet had the choice whether or not to click on the thread.
I am sure it was not maliciously done - I suspect she was excited by the final, and couldn't wait to discuss the outcome - nothing wrong there - so started the thread and genuinely didn't think about the spoiler in the title.
But knowing she's spoiled something for others, and being so blase and don't-care about it, is not pleasant to see. The decent thing to do would be to apologise.
Yes, it is just a TV programme, and yes there are other spoilers out there, and no, it is not the end of the world - but people have been following this for weeks, building up to watching the final and finding out the result at the end of that, and to have the result revealed in a spoiler is a disappointment for them.