I find public speaking "easy". As a teenager I spent a couple of weeks in front of rowdy groups of hundreds of school students during school election debates for a fringe party that made me an ideal candidate for mockery. Didn't phase me. I held the commencement speech at my university my first year in front of thousands of students and TV crews, and people couldn't believe how relaxed I was, but I didn't understand why there was anything to be nervous about; I knew the manuscript and was after all just going to stand there and deliver it. I've spoken to quite a few very varied large groups of people. I don't react to that at all.
Smalltalk with strangers, on the other hand, takes a lot out of me. It's not just that it triggers anxieties in some situations, but even when it doesn't, it is exhausting: I have to concentrate much more to actually listen and take part in conversations that happens for "social reasons" as opposed to about specific subjects, as otherwise I miss cues etc. or simply will go silent.
I don't mind getting up in front of a crowd of any size without a manuscript and deliver my message, even if I know it'll be unpopular, but if I'm in a store, for example, I'd rather spend a couple of extra minutes looking around rather than asking someone who works there to help me find something, because one on one interactions with strangers are draining. I even have to consciously avoid taking steps to "bypass" people I only know vaguely but who I expect might want to talk to me as if I just go on "autopilot" I'll pick the route that leads to the least amount of words exchanged.
Perhaps the "aren't going to talk back" part is what makes public speaking feel easier for me, even when they actually do.
To me, those two things are entirely separate. I'm sure there are lots of introverts that also have problems with public speaking, but conversely I know plenty of introverts other than myself who doesn't have a problem with public speaking, and lots of extroverts who do.