Nobody really knows but it was reported by enough credible reporters that it is most likely the case.
I get it, it wasn't simply, "since Velasquez might be injured, give us Appel, who is worth more than Fisher" (though he was). It was also, "since Velasquez might be injured, we want even more pitching."
One thing is for sure, Klentak hasn't been wrong about needing as many pitchers as possible.
And the Phillies also already had Williams/Altherr/Quinn, and still do. If Fisher turns out to be significantly better than Williams that will be disappointing, but as much because of Williams as because of not having Fisher.