If Clark was apparently time traveling, why didn't he...stop the assassination that he would have therefore known about? I guess time travel reasons, but on the whole, the exchange between him and JFK doesn't read as a scene between someone who knows the outcome of events and one who doesn't. Or that Clark seems at all out of place in this time period.
It reads as JFK trying to convince Clark who has been operating in the shadows in 1963 to announce himself to the world. In 1963. And it honestly seems as if Clark is experiencing things from the perspective of the 1960s, i.e. that he doesn't know any other superheroes exist yet, as JFK is telling him that there are others out there. A Clark from our time period would be like "yeah, I know." It also seems that Clark is genuinely asking Kennedy if he needs his protection in Dallas, as in, it doesn't seem like Clark KNOWS what's gonna happen in Dallas. If Clark were really time-traveling here, he'd know that JFK dies.
The King Arthur line is just a throwaway to explain why he has the table.
Again, if what you're saying here is true, great, but what you're pitching requires a LOT of filling in the blanks on the part of the reader to the point where it's almost inventing an explanation. In other words, if this issue had been better-written, it would come across as "Clark is time traveling" without having to straight-up say "Clark is time-traveling." Instead, what we have is an issue that doesn't seem to care if the audience does or doesn't know what is happening, as it does almost nothing to provide any sort of context. That's...not good writing.