That seems likely to alter chemical behaviour (as researchers found). I'm not a chemist, but it seems reasonable to consider compounds with such bonds as different compounds. Why should we think of carbon-12 and carbon-13 as variations of carbon, instead of distinct elements - if they have different chemical behaviour?

The blackbox testing tells us that 35% heavy water is lethal, but doesn't tell why. It's possible - and even likely - that it is the very bonds we wish to protect that become lethal if strengthened 80 times.

The final "heavy babies" grayed paragraph at the end is fascinating (in case you skipped it: babies have more carbon-13, and their mothers are unusually depleted with it around the time of birth.)