This looks like some lazy reactionary teenage contrarianism. Similar to what I have been seeing from some celebrities this week.
1) Nothing really at stake. Traffic from Twitter probably close to zero for The Guardian (as for any media company)
2) I'm 120% sure most of the editorial stuff will keep posting and/or spending hours a day there
https://www.engadget.com/apps/the-guardian-is-leaving-x-144549755.html