Sounds like something their lawyers made up, perhaps?
The first comment on the post has a pretty relevant paragraph, which I think sounds plausible: Section 3(b) allows you to provide a written offer for source. I think HTC is interpreting this to mean that if you respond to their written offer for source, there's obviously going to be a delay for them to get your written request, put together the source code and send it back to you, and they've decided that 90 to 120 days is a reasonable amount of time for that.
The commenter also says it seems this is pushing it, and I agree - but HTC's lawyers clearly think it's worth the risk.
