I would agree that if there hasn't been an explicit "let's get married" or "will you marry me?" from one and a yes from the other, the two people aren't engaged.
What seems weird to me is when Alice and Bob have had that discussion and agreed to get married, and have told other people that they are planning to marry each other, and then Alice (or Bob) says something like "we're still planning when/how he's going to propose to me," meaning "when we are going to do this particular shape of sentimental." If Alice and Bob have had that discussion and not told anyone, that's a gray area, and it's their relationship, not mine. If you are both telling people that you have agreed to get married, it seems odd to also be talking about the "proposal," so called, as an event that will happen at an indeterminate future date.
It seems weird to me, too. For example, a friend of mine and her BF had many serious discussions about marriage, agreed that marriage to each other was the next step in their relationship
, began plotting out a timeline and other plans for the wedding, and went shopping together and purchased rings (and told me, and other people, about all of this). But, she did not consider that they were actually engaged to be married, because he had not yet made an official proposal in which he presented her with the ring and asked her to marry him, as part of a planned evening with dinner at a nice restaurant, etc.. And then after that happened they called everyone and announced that they were engaged. (And I have to admit I was like, "Oh, that's great. But I thought you already were? You asked me to be a bridesmaid the other day..."
Honestly, it doesn't matter to me either way, because it's not like her attitude is hurting anyone else. I just don't personally share it. I'm pretty low-key about some things and if I couldn't pinpoint the exact date and time that I "became engaged," because it happened at some point in between agreeing it was the next step and actually starting to make a wedding a reality, that would be fine with me. If the person I had agreed to marry started making noise about how we "weren't really engaged yet," I would be looking at them nervously and saying, "Wait a second, are we getting married or not? I thought we agreed? I've been writing out guest lists and looking up 'change of marital status' forms at work! Am I jumping the gun here?" And if they said they wanted to have a big proposal "moment" I have to admit I would think they were being kind of silly. But, that's probably unlikely to happen, just because if we were getting married, we would hopefully know each other better than that, and be on the same page about such things.
But like I said, it really doesn't affect anyone else (unless the couple is roping in unwilling third parties or something), so neither attitude is rude. But, to go back to the original subject of the thread, my opinion on this is why I personally don't find (other people's) public marriage proposals to be especially enjoyable to witness. I wouldn't ask any third party to not do one if they both wanted to, or rudely interrupt them somehow, but I would be one of the people quietly walking on by, not part of the crowd standing there watching and applauding.