Whether this is true or not, you cannot make a magic item on the presumption that there will be no minions past level 10. Minions or not is a campaign decision. There are minions of all levels in the MM. There are styles of campaigns that want plenty of minions at all levels, such as a military campaign. If the rules do not support such campaign decisions, the rules are flawed. This seems to be one such flaw, and it is good that it is pointed out.
The rules do support those campaign styles. And they also support a DM who says 'Listen, there's a ton of minions in this campaign, and I am not comfortable with how that blade interracts with them in that environment.' Players are not automaticly entitled to this weapon just because it's in a book.
But more importantly, the assumption is that a DM designs his encounters with the powers and items of the players in mind. So if you have one of these items in there, and players have abilities that make minions into worm food quickly, then you stop using minions regularly as challenges and you adjust how you use monsters when you want to challenge the party. Like you do for -every item or power- in the game.
And, if they have this weapon, once in a while you actually open up the floodgates, and you toss forth an army of minions, and you let the players just go hog wild with this weapon, the minions, and some of their larger friends. You don't do it often, but you do it once in a while when you want a crowning moment of awesome.
This is a part of the DMs responsibility to the game, and you can't expect a book or game designers to do this job for you. You cannot reasonable expect game designers to come out with weapons that simultaneously appeal to all styles of gamers and games while being balanced in all styles of games and for all styles of gamers. That's just not possible, and not even desirable to try.
