Sadly, that is the most depressing part of the whole story in my opinion. If they were just intending to make Glimpse "GIMP but with a different name" so that they automatically inherit all future work on the upstream code then I would not have a problem. But no, they've already announced plans to split the developer base in two by diverging from upstream.
(You may argue that such plans don't, in themselves, split the developer base. But the number of willing FOSS developers is a finite resource and I'm struggling to think of a case where some took an existing FOSS product, forked it, and increased the amount of work going into either branch post-fork. On the other hand, I don't struggle at all to remember counter-examples like LibreOffice, which survives only because OpenOffice so very obviously died on the vine within a few months of the split.)