His employer is not the issue, he failed to comply with a Congressional subpoena.
MacCracken was on several occasions held in custody by the Sargeant at Arms of the Senate. After MacCracken had been released and upon a new warrant being issued for his ignoring a subsequent subpoena, the Sargeant at Arms reported Feb. 12, 1934 that he went to MacCracken’s place of business to arrest MacCracken but he was unable to locate him as MacCracken was in hiding. 73rd Cong., 78 Cong. Rec. 2410 (1934) https://archive.org/details/congressionalrec78aunit
This is a better link.
https://archive.org/download/congressionalrec78aunit/congressionalrec78aunit.pdf
Go to pdf page 1331 (printed page 2410) at the bottom of the left hand column.
The quote below, which I assume in good faith came from the 1800+ page document you cite, says the Sargent at Arm, his own self, went to the man's place of business to arrest him but could not as he was not there.
However, I'd guess without citing any documents, that it was actually a US Marshall who went to serve the warrant (under the auspices of the Senate and Sargent at Arms) because the Executive Branch was supporting the Congress in this matter. However, when the Executive refuses there is no enforcement unless a Governor, Mayor, Sheriff etc. decides to support with enforcement.
To wit: "John Marshall has made his decision; now let him enforce it!".