It can be both true that the situation called for having a co-founder put on leave, and his wife banned from the premises because a first look shows Horvath's claims are likely to be true and that Github's management deserves respect for prompt action, candor, and open communication.
They could have been completely opaque and done nothing but investigate the matter, without any disclosure. It's a brave choice to be open because, even implicitly, admitting a problem can taken as admitting liability.
As many others have said in this comment thread, Github's actions do NOT mean any claims are "likely to be true". It just means Github is taking proper steps to investigate the situation. The investigation may lead to them determining that Horvath, and not other employees, was in the wrong.
At the very least, she seems a bit...attention hungry. Knowing nothing about the situation or people involved I am taking her accusations with a huge grain of salt.
They could have been completely opaque and done nothing but investigate the matter, without any disclosure. It's a brave choice to be open because, even implicitly, admitting a problem can taken as admitting liability.