That thread was clearly one where they were using <i> italics tags intentionally - keying them in themselves - and then removing them afterwards. That's a way to work around a limitation in InDesign's indexing tool. Is that what you are doing?
Upthread from here, FRIdNGE suggested a regex-based solution. If you just need to remove the <tags> and leave italic text behind, his suggestion is really close to what I'd do. Instead of a GREP Style, which (as you note) can't remove the tags, and instead of hiding the tags, which is suggested in that other thread, why not just use a normal GREP query to yank all of the tags all at once?
Find: (<i>)(.+?)(</i>)
Change to: $2
Style: your preferred flavor of obliuqe/italic/italic character style/etc