I doubt 50% of Fortune 100 customers are all in on GitLab. My guess is most of their F100 customers are acquisitions that were using Gitlab and continue to use Gitlab.
When my company was acquired by $MegaCorp, I noted one of vendors was like "trusted by $MegaCorp" because yes technically, they got a check from $MegaCorp but $MegaCorp was not interested in becoming further customer.
Or one or two devs in the F100 customers made an account using their work email so they could chuck some OSS prototype code somewhere, or test something out.
When my company was acquired by $MegaCorp, I noted one of vendors was like "trusted by $MegaCorp" because yes technically, they got a check from $MegaCorp but $MegaCorp was not interested in becoming further customer.