I had a problem at a tourney this week because someone used bottomless and I chained with my decree. I know I can chain decree, but this is what it says at yugioh-card:

You CANNOT activate (flip face-up) a Continuous Trap Card and also use its effect in the same chain. In order to use the effect of a Continuous Trap Card, it must have been activated (flipped face-up) before the current chain was initiated.

What reasoning do I need to use to tell him that decree can be chained to other trap cards and still use its effect?

The most logical reason is that since Royal Decrees effect is continous, there is nothing triggering it no manual effect, there is nothing else than the effect coming into play immediately after succesful activation.

Chains resolve in reverse. Your chain was:

Bottom Less Trap Hole -1
Royal Decree - 2

As long as Royal Decree was activated in response to the activation of BTH in the same chain Royal Decree's effect is applied first negating the effect of BTH. Continuous or not does not affect this matter. It was the same reasoning for IMperial Order when it was flying around.

Of COURSE it matters, because you CANNOT use a card like Ultimate Offering effect inside a resolving chain, even if it activated succesfuly, Royal Decree has no activation requirement, it applies immediately because it has a continuous effect.