The wording is pretty key here, it's an easy thing to miss!

Giving someone a plus 1 modifier means you still have a 3+BS (in this case) but you can hit on the roll of a 2 (2+1 = 3). If you suffer -1 to hit for any reason, you hit on 3s again.

Increasing 3+BS to 2+BS means you hit on twos natively (there's no modifier added anymore). If you suffer -1 to hit, you hit on 3s here too.

However, with both combined, you hit on 2s natively AND if you suffer -1 to hit you still hit on 2s as your +1 to hit cancels it out.

A natural roll of a 1 always fails, regardless of any modifiers, so failure is still possible unfortunately.

If warriors had +4BS however, they would both stack to give them 3+BS and +1 to hit, so they'd hit on 2s.

I hope that makes sense. Essentially, modifying the stat block and modifying the role are two separate buffs that can work together while stacking modifiers to the roll generally does not (unless it's armour saves / AP)