It's just standard terminology. See
Sign Gifts
But if you want further reasons specifically for the testimony at hand (post #1):
HELPS Word-studies
4592 sēmeíon –
a sign (typically miraculous), given especially to confirm, corroborate or authenticate.
4592 /sēmeíon ("sign") then emphasizes the end-purpose which exalts the one giving it. Accordingly, it is used dozens of times in the NT for what
authenticates the Lord and His eternal purpose, especially by doing what mere man can not replicate of take credit for.
In the testimony, the healing of the 7 deaf confirmed the preaching of the gospel.
I highly encourage you to watch the testimony.