Immigration can lead to less competition and therefore innovation. Instead of people from other countries creating services that compete, they get absorbed by multinationals.
Is it? I know a lot of leftists who consider corporate profit margins something to intrinsically oppose. I mean, they're concerned about the consequences of those margins, but they do think the consequences are inescapable - they think rising corporate margins inherently make the world a worse place.
And yeah, that's historically driven an anti-immigration streak among labor movements, made stronger when it's not about new citizens but people operating under alternate labor laws. The deBoer type leftists of today don't carry that xenophobia, but some of them definitely still complain about H1B as a corporate subsidy.
It's not a position I endorse (if anything, I'd argue for "strengthen the bargaining position of H1B workers"), but I don't think it's terribly uncommon. I agree that the comment framing is inflammatory and unnecessary, but I wasn't surprised by the part about margins.
What I think you mean is that you can be against increased margins for corporations, and that's the reason that you're against immigration.
It's a strange strange thing to be against increased margins for corporations, without any other explanation, so I can see why many people would be confused, if that's what you're trying to say.