The question presupposes an error: not all the money in circulation is a liability for central bank.
Your statement is correct... except for the part of new money "loaned out to the commercial banks by buying treasury bonds". The new money created by commercial (i.e. non central) banks is backed with their own deposits in central bank just for the percentage defined by "fractional reserve requirements" (or the precautionary reserves decided by the bank itself, if higher). [There is another misstatement with the expression "by buying treasury bonds", but it's outside the point here.]
So, the money in circulation (ambiguous expression itself, as long as it could refer to different measures or categories of money: google m1, m2, m3) is a central bank's liability only in the amount of hard cash (M0) plus the part of the current deposits which the commercial banks, in turn, keep backed by deposits in the central bank.
As a proper accounting question, it's almost a definitional question: only "the money" which is backed by central bank liability sums up as a central bank liability.
The rest of the money issued by commercial banks is backed by their own liability. In fact, all the money issued by commercial banks is backed by their own liability, but the fractional reserve kept as deposits in central bank are assets that offset the gross amount.
Additionally, and contrary to what we commonly think, those liabilities do mean that banks, central and commercial, owe those sums, even though after the end of convertibility nobody will, nor can, draw the credit from the central bank, at least not in other form than in current money itself (returning to initial position). Anyway, those liabilities could be claimed from a bank (commercial and, theorically, also central) by transferring the deposit to another bank, which causes the liability itself being also transferred from one bank to another.
Finally, we can answer your question: money in circulation is a liability for bank system (central as well as commercial banks), from the point of view of bank balance sheet. Interestingly enough, that is why creating money doesn't mean creating net wealth at global level, as an asset and a liability spring at once.