Its similar to the android API google turned off that let an app check what other apps were installed, that was used to fingerprint and that got removed. Same deal here.
Can browsers defend against this with technical countermeasures?
Considering this is only an issue on Chrom*-browsers because they are the only ones offering the API used by LinkedIn for that scan...
EDIT: As explained in the original submission when this was detected, Firefox is randomizing extension IDs per site, Chrome does not.