You could say the same thing about the indent. Sure, it's annoying, but it's not super hard to fix. It's not like these guys are cranking out 125 titles every week. So what they're saying is you could send them the next Harry Potter, but they're not gonna read it because two spaces between sentences. That's just stupid.
Actually, it's not. A successful publisher (of any size) will always receive more work than they are able to take on. Making your manuscript harder to format is a good reason for them to pass for someone who knows how to do the work on their end. An author not doing the work is like showing up to an interview in your pajamas - it shows a lack of respect for the publisher.
And, while it seems like two spaces after the period is "simple enough to fix" on the publisher's end, that attitude quickly rolls out into not using the correct fonts, or putting in weird colors, or failing to write a proper query/cover letter. All of it demonstrating an author who is likely to be difficult to work with because they either can't or won't follow simple directions. Or who wants complete control over the product. (Which, btw, is fine if you aren't working with a publisher. But if you're wanting to work with a publisher it should be because you feel they have something to offer that you don't, which means working together and respecting what they bring to the table.)