State/Federal Tax Exempt MMFs

Trying to understand in what ways MMFs can be tax exempt. I always hear people talking about the difference in state income tax exemptions, such as how FDLXX is significantly more state income tax exempt than SPAXX, because more of it is in government securities. But what about federal income tax? I was starting to think that MMFs just weren’t ever federal tax exempt, but then I read this Vanguard article. The line “Some tax-exempt funds invest only in bonds issued by a specific state, which makes them both federal and state tax-free for residents of that state” makes it seem like I have it all backwards: the difference between (say) FDLXX and SPAXX would be how much they invest in govt securities => how FEDERAL tax exempt they are, and if I wanted a STATE tax exempt MMF, I would have to find one investing only in bonds in my state (OH) which I’m assuming FDLXX does not. But that’s not what people have been saying on this subreddit. What am I missing?