Now granted... this is all just getting hung up on the fluffy item name "of Speed" as to determining whether a rule was intended or not. But there are other places in the game where there are similar mechanics but are not fluffed in the same "speedy" way. For instance... the War cleric gets to attack with their main hand weapon with their Action and their Bonus action when using its War Priest feature.. and the second attack isn't fluffed as being "speedy", it's just part of being a War Priest. So the "of Speed" of the scimitar doesn't mean you have to necessarily interpret the intention of the magic item property that way just because the fluff seems to indicate it. It certainly makes sense if you do, but there are other places where similar things happen so you don't have to.
I think my thing is any character can already attack as a bonus action with any scimitar simply by attacking with another light weapon first. So if you have to take the attack action it is not very powerful and it should not be very rare if you need to take the attack action to use it.
The rewrite like Paul mentioned would fix this, but I certainly don't think giving an extra action is RAI.
I also see the Warpriest feature or Two Weapon fighting thematically much like a boxer throwing a 1-2 punch. The first attack sets up the bonus action attack, letting you land an extra blow in there and the "speed" of the Scimitar of Speed is what allows you to sneak it in without that.
The reason I brought it up to start with is it is very rare to let a PC attack with a bonus action if they did not attack with an action. Most things do work like the Warpriest feature. As a matter of fact the only class, subclass, race or feat feature that I can think of that allows a bonus action attack without an attack action is the Quick Toss maneuver.