Catering would be explicitly dealing with issues for minorities(LGBTQ included) that are issues for the wider population without acknowledging it in that context and often without attempting to help the wider populace.
One of the examples that's extremely common is the idea of helping out poor black communities while poor whites often get no attention at all. The problem there isn't that black people are poor, it's that anyone is poor. Rising tide lifts all boats, and all that. Setup the minimum safety net for everyone, and make sure there are no holes to fall through, and that solves a lot of problems and makes it so that dumb people who are angry at welfare systems from a racial perspective have a much less sturdy soapbox to stand upon.
Looking at the complaints on
https://www.queersagainstpete.com/letter(which was linked in the guardian article referenced earlier, and I hadn't actually read until this point) I don't see any one specific complaint that's directly related to being LGBTQ. It's all generic "not progressive enough" complaints that connect to LGBTQ via questionable intersectionality logic. They even acknowledge that he's for the Equality Act which is the one thing that IS directly related to LGBTQ issues that's addressed in the letter.
You can complain about things like(taken from the linked letter):
*universal free public college
*cancelling student loan debt
*restore the right to vote for all formerly and currently incarcerated people
*the concerns related to Eric Logan
*a moratorium to end deportations
*decriminalize border crossing
*Medicare for All and universal childcare
*demolished homes
*boycotting for political reasons
*cap credit card interest rates
*guarantee a job to everyone who needs one
*increasing of defense spending
Those are all legitimate issues to be worried about. And issues that if they bother you enough are reasons to not vote for Buttigieg.
But they are not LGBTQ issues, and framing them like that makes it seem like every LGBTQ person has to be on the side of the people who wrote that letter and completely silences LGBTQ people who might be more moderate or even conservative.
So, TL;DR: In this particular case, catering would be catering to left wing progressives when he's very obviously trying to be a moderate candidate. It just doesn't make sense, would alienate his current base, and he'd be fighting over the wedge of the electorate that already hates him for stupid reasons they aren't likely to get over from a few "promises" and already have a candidate they like.
I admit, when I said "cater to" I saw "protester" and assumed they were after something crazy, rather than just talking normal policies in a somewhat dishonest way. The article doesn't really describe what they were angry about except in general terms and that the fundraiser tickets were too expensive. In general though I'm talking about the push for specific legislation that sets apart LGBTQ as a protected class. If a law is good enough for LGBTQ people, it's good enough for everyone. And if for some reason the law protecting everyone isn't protecting LGBTQ people, that's an issue with the enforcement or implementation of the law that needs to be dealt with that a new law won't help. The Equality Act is in general a good example of a law(not perfect, but good), since it protects a gay person from not getting a job or a house because they're gay as much as it protects a straight person from not getting a house or a job because they're straight. I'm fine with the law being used more often in one direction than the other, but I'd be glad that it's there in case I do ever need it and it would piss me off if it didn't work in the other direction as well. Trying to set some sort of affirmative action for a specific group while ignoring others who aren't in that group is just begging for increased divisiveness.
We really should just be trying to help each individual that needs help, regardless of who they are or what kind of category or label you can put on them.