I expect you can find both extremes, and maybe everything in between, for example:
can a Buddhist see another spiritual tradition as valid, even if it is not Buddhism?
I think that's a more difficult question -- maybe it's easier to say that another person isn't bad, but harder to say that another path is valid.
One of the characteristics of the Buddhist Dhamma is that it's something that you're meant to "know for yourself" -- perhaps it is impossible to personally know another spiritual tradition in the same way that you'd know the Buddhist tradition, so maybe you'll always be saying "they're not the same path, they don't lead to the same destination/result" and so on.
On the other hand I think that Buddhism has some definitions of what "good" is -- for example kindness, harmlessness, equanimity, faith, selflessness, strength, non-suffering, liberation, and so on and so on. So I think that to some extent any person with these qualities and any path which promotes these qualities might be seen as admirable or at least beneficial.
That's maybe unlike the stereotypical extreme "fundamentalism" of another religion, which might say something like, "God defined rules X and Y in book Z and anyone who questions or disobeys is therefore wrong".
all other spiritual traditions or sects are false?
If you look into the Buddhist scriptures for justification I think you can easily find passages which support both arguments (i.e. arguments for and against other traditions being "false"):
- Passages which praise the Buddha as being right, and the Buddhist Dhamma as unexcelled, and so on (which might be taken to imply that, to whatever extent other paths aren't Buddhism, they're less good)
- Passages which warn against conceit, against sectarianism, even against attaching to specific views