The presentation of the data doesn't support the conclusions drawn, i.e. women have been actively impeded in songwriting - they might have been, but the data doesn't say that on its own.

Also, some credits were only there due to sampling, the songwriters did not actively sit down and write or co-write that particular song.

Given that music has been almost completely democratised with the lowest barrier to entry there has ever been, it's hard to say women are being prevented from songwriting, but breaking into the highest echelons of the hit industry (a dubious accolade) might be tougher. We don't know how many are trying.