Hriston
Dungeon Master of Middle-earth (he/him)
It can be understood that way if you ignore how things are written. It doesn't say "If you are going to take the attack action on your turn." or "If you take the attack action at any time during your turn."
Those both seem like acceptable glosses to me. I'm not sure why you and Jeremy Crawford regard the existing language as so restrictive. The way I conceive of game elements like the first bullet of Shield Master, which grant you a bonus action, is that they let you do something extra, a bonus, that other characters who don't have that game element can't. In the case of Shield Master's first bullet, it lets you do your usual attack routine plus shove a creature with your shield. It costs you your action and bonus action for that turn. That seems to meet the design goals of the feature, and I don't see any benefit to the game of being more restrictive than that.
It's written in the "if, then" format. "If you take the attack action on your turn," which is the "if" portion, "you can use.." It's classic "if, then." The word "then" not being explicitly written doesn't alter that.
Yes, it's called a conditional sentence. It expresses the implication that if it's true that you take the Attack action on your turn, then it's also true that you can use a bonus action to shove a creature with your shield. But this is natural language, and I think it's a mistake to read the word then as implying a specific chronology (as your emphasis with all caps seemed to do), especially when you can express something like this sentence in English:
If it rains this afternoon, then yesterday's weather forecast was wrong.
Differently than abilities such as Two-Weapon Fighting which is written differently. Two-Weapon Fighting just needs a single attack to happen first.
I'd say it works the same way. You can use a bonus action to make an attack as long as you also take the Attack action and satisfy any other conditions. It's the same structure that governs all these features.
It's a shame that he shifted his focus. It was great to know what was intended and not just the literal interpretation of RAW.
I think it's telling that he included Shield Master's bonus action shove in a category of "most bonus actions" that he said did not have a timing specification.