Your post confirms that the Guarantee is unsecured and subordinate - in the event of a bankruptcy no payment can be made on the Guarantee until senior debt is paid in full.
From the prospectus-
Status of the Guarantees
The guarantee will constitute an unsecured obligation of Lehman Brothers Holdings and will rank:
• subordinate and junior in right of payment to all other liabilities of Lehman Brothers Holdings,
• on a parity with the most senior preferred or preference stock now or hereafter issued by Lehman Brothers Holdings and with any guarantee now or hereafter entered into by Lehman Brothers Holdings in respect of any preferred securities of any affiliate of Lehman Brothers Holdings, and
• senior to Lehman Brothers Holding's common stock. The guarantee will not place a limitation on the amount of additional senior debt that may be incurred by Lehman Brothers Holding