Does an enquiry (merisik) already count as a proposal in Islamic law?

No; merisik is only asking after her and stating an interest, while a proposal happens when the wish to marry is stated clearly. In Malay custom, merisik is an early step to find out whether the young woman is not yet spoken for and whether her family is open. At this stage, neither side is bound. The prohibition on proposing to a woman who has been proposed to by someone else applies once that proposal has been accepted, not merely because a family came to ask. Even so, be courteous. If you know another family is making an enquiry, wait until there is a decision. See proposing to a woman already proposed to.