Input/output pin resistance for an AVR microcontroller
It's right there, in section 21. See it?



\$\frac{0.25\text{V}}{10\text{mA}}= 25\Omega\$



\$\frac{5\text V - 4.75\text{V}}{10\text{mA}}= 25\Omega\$

Therefore yes, the output has a resistance of 25\$\Omega\$ at 25\$^{\circ}\$C.