Biden appears to be playing his cards diplomatically, by not responding to the 'indestructible' comment (compare and contrast to Trump who offended half of Europe's politicians before he even landed).
For its part Johnson's comment looks merely like another total untruth told in a desperate bid to make things look better for himself and his clueless ministers. In reality I'd imagine Biden thinks of Johnson as a poor-man's Trump (in fairness to Johnson, I don't think he quite aspires to the full-package, though he does have his moments) whom the Democrat govt in Washington is keen to get out of the way before a more sensible and trustworthy replacement is ushered in. In the mean time Johnson better play nicely re the EU/N Ireland so that Biden doesn't have to say anything overly embarrassing thus endangering the 'special relationship' UK govt's claim.
Johnson's govt doesn't even have much in common with 'proper' Conservatives/Republicans, never mind Democrats, I don't think it would be over harsh to consider Johnson as just a bit of shit on Biden's shoe.