Mercedes undercut chance was scuppered before even the outlap. If you look at the telemetry and the lap times for the in lap, and each out lap, if the positions were reversed, and Max was the one boxing to undercut, he would have done so pretty closely, even ignoring the outlap.
Lap 50 - Which was Lewis' in-lap was a 1:35.209 (thats the lap that includes driving into the pitlane and box. And the stop itself presumibly).
Lap 51 - which was Max's in-lap was a 1:33.905 (again, the lap including the box and the stop) Which is 1.304 seconds faster over all, before any out-lap times were taken into account.
Closest Lewis was to Max during the start of his inlap 1.395 and actually increased to 1.6 to 1.8seconds on the actual inlap, rather than closing right up.
Lap 52, which was Lewis's out-lap was a 1:19.702
Lap 53 - which was Max's out-lap was a 1:18.724
Thats a difference of 0.978 across the out-laps (so basically a second).