Are PBGC payments for a failed TWA pension considered to be "federal government reitirement benefits?

No.  PBGC is just the backup for Private pension systems....and payments are not considered as Fed Govt Retirement benefits.

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Federal Govt retirement benefits, refers to payments to people who actually worked for the US Govt. 

In general, Pension Benefit Guaranty Corporation (PBGC) payments for a failed Trans World Airlines (TWA) pension aren't considered federal government retirement benefits. The PBGC is a federally established agency that insures private-sector pensions, not public sector or government employee pensions. 

Federal government retirement benefits come from the Civil Service Retirement System (CSRS) or the Federal Employees' Retirement System (FERS). 

For more information, check out the IRS Publication 721