Usage of "as if" as interjection

My question concerns the usage of "as if" as an idiomatic interjection. The references I have consulted (etymonline, online dictionaries, etc.) seem to agree that "as if" is used in this context to convey incredulity or doubt. This is how I have always thought of it.

The other day, I was with friends who used "as if" to express their surprise and joy about a wedding proposal. They weren't being sarcastic or incredulous; they were genuinely happy about the proposal. Here is a loosely paraphrased version of what they said:

"As if they're getting married!"

Is my friends' usage of "as if" standard? Is it a primarily regional distinction? (I am in Ontario, Canada right now.) The closest example we could come up with is someone says "I can't believe that you won first prize!" Of course, the person can and does believe it, so their words ought not to be taken literally. Is it the same idea with "as if"?

The OED’s only entry for the colloquial use of as if is ‘Without clause, as an exclamation. Typically used as a sardonic response to a stated or reported suggestion’. Like you, I have never heard it used in any other way. You say that As if they're getting married! is ‘a loosely paraphrased version of what they said’. Did they perhaps actually say something different? They could, for example, have meant that there was so much celebration of this good news that it was as if they’re getting married already.

The phrase as if is used extensively in the US, especially among the teenage and very young adult population.

It is used sarcastically, denying either the preceding statement or the statement that follows.

James and Jodie dating? As if!

As if I would ever go out with you.

The actual intent of the phrase is to convey It is not as if X is happening.

According to Urban Dictionary it means

YEAH RIGHT! [but sarcastically]
Made popular by the movie CLUELESS staring Alicia Silverstone. Meaning: I'm definetly not going to do that / not into that/ don't beleive that / etc. If a guy tells me he knows I'm in love with him (but I think he's a total loser), then I say to him "AS IF!"

In the US it would not be used in circumstances where the speaker agreed with or believed the associated statement.

I recently moved to Ottawa, Ontario from New Brunswick, Canada, and was extremely confused with the usage you described above. Everyone I talked to had "as if" meaning "I believe it, but am shocked or incredulous at the news".

You get news that X is going out with Y, a surprising but plausible scenario. "As if X is going out with Y! Good for them!"

Whereas I did not have that meaning AT ALL. For me, "as if" has only one usage of "I completely do not believe the statement".

You get news that X is going out with Y, a completely impossible scenario that you do not believe for a second. "As if X is dating Y! Stop spreading rumours."

My friends' usage was completely confusing to me.

I first encountered it in writing, and I was genuinely hurt because I thought the other person did not believe me. Later, when I spoke to him, he explained that he DID believe me, but he was just shocked at what I had said.

I did a quick poll, and the people back in New Brunswick that I asked only had the "doubt" definition and not the "surprised" definition. I have also been wondering if it's a regional expression. Where in Ontario are you seeing it?

Only in the present tense, I imagine.

Look at Mary in her white dress, staring at John while he puts that ring on her finger; as if they're getting married.