The actual offer states [0]:
>As a result, I am offering to buy 100% of Twitter for $54.20 per share in cash, a 54% premium over the day before I began investing in Twitter and a 38% premium over the day before my investment was publicly announced. My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder.
Which means he is asking for approval, which seems to contradict the headline and the quote from Mirabaud Equity Research which was used to make the headline more sensational as they were quoted saying “This becomes a hostile takeover offer which is going to cost a serious amount of cash”.
If the board disapproves of the offer and he acquires a larger stake of ownership, that would be more inline with what a hostile takeover is.
[0] https://www.sec.gov/Archives/edgar/data/0001418091/000110465...