If the taxis are doing a strike, you can speak of strike breaking only for taxis not participating in it. Uber are not taxis per se, so there is no way they can break a strike they aren't doing in the first place.

Also there seems to be a confusion made on strike and strike consequences or demonstration. Being on strike only means stop working, not directly blocking access or service (although it is often done in conjunction). So again Uber providing service for people not serviced by taxis can not be labeled of strike breaking: Uber didn't prevent any taxis to be on strike if they want to do it.

Aside my opinion on the matter as a French who suffered more than one time from strikes is that Uber did a good job by allowing people to live their lives normally. Not being able to move, send a mail, study, lend a book, ... is really frustrating.