I think this might be more common than folks admit. I look at it that you are invited to be part of the celebration of marriage. I would hope they are inviting to to be there because they want you there, not because they expect an expensive gift. Traveling to weddings can be very expensive and some might have to give a smaller gift to afford to go. Sometimes it's not even just travel, you might need care for children, you might have "dress expectations" and honestly some involve taking off work. And I'll add

sometimes economic losses happen when there is a wedding summons with a month notice ...
Well yes the ceremony and union is all about the bride and groom. The celebration part involves being surrounded by the people you feel are important and care about. The bride and groom who make decisions that put work/expenses on a guest or perhaps exclude them entirely have to own their choices. AND that is okay! Just don't complain if it changes the outcome.
That doesn't work for a COCKTAIL (OP) or Black Tie dress code.
Even low cost Tuxedos are not cheap. Even cocktail dresses on clearance are not cheap, and if they are on clearance they are often not the right size. Most folks get an invite 4-6 weeks out and that isn't much time for many to find something they can afford, especially if they have travel expenses and a gift.
*I have NO ISSUE with dress codes at weddings as long as it fits the venue situation.

If she only wears purple then anyone who invites her anywhere knows what she is wearing.
I agree not a bad request, I think the bigger question as I stated before is SUMMER COCKTAIL BACKYARD - I need to know where is this located, what is the setup, will there be A/C in a tent etc. Before I go out and buy a cocktail dress that can be reasonably dry cleaned for ONE night, I need to know I won't be miserable.
Sadly Miss Manners left the room long ago. Folks don't seem to understand that. I was upset when DD wanted to do online RSVP vs a card. She said online reinforces who is invited and restricts how many you can RSVP. It will be their 2nd reminder that kids are not invited.
Actually based on many reports across social media (and personal experience) there are folks who do consider them summons. There will be those who will have a fit if someone doesn't follow the rules or doesn't come at all. Just like there are some who think their kids are required to be invited, their cousins should be invited, etc. Again personal experience.
I don't think it's always a video photo instagram thing. DD wanted a lovely adult evening, a treat for some (and it was appreciated) and a beautiful memory. She wanted folks dressed up. There was no video, she didn't post a bunch of photos. She wanted her special day to feel special, not just another gathering.
All she said was semi-formal. Most came dressed up. A few were business casual. And one cowboy.
I do think OP situation might be just so guests don't look like wedding party. DD selected a company that offers lots of styles in same fabric/color. She told her girls whatever they were comfortable in style wise and choose a price point (it varied but not too much). She told them whatever shoes they wanted was fine. Didn't want her girls to spend too much. One girl was short so she chose tea length, rest long. What was funny is there were a few guests in the same exact color and even I did a double take since their dresses weren't all the same. Maybe she wants to avoid that.