I think given that he tweeted "results soon..." or whatever before he knew what he ran, then deleted the tweet he he saw that he finished in 20:25 chip time indicates that he was hoping to run sub-20 and even thought that he did after "finishing".
If he was shooting for a close-but-not-quite sub 20, then I don't know why he would have deleted the initial tweet and you'd think he'd mention his time in some kind of braggy way.
To me, there are three possible reasons that he didn't get the sub 20:
1. Screwed up the math. Most likely possibility. Based on what I've seen out of Mike, he seems to have the intellect of a 5th grader. However, I guess his math skills are even worse than that.
2. He wasn't able to covertly sneak back into the race course in time to hit sub 20. He knew when he needed to cross the line, but it was too risky at that time and once the coast was clear the 20 minute threshold had come and gone.
3. He did the math right and jumped in at the right time but he overestimated the speed he would be able to cover the remaining route in to get under 20 minutes. So, if he jumped back in with a quarter mile to go and thought he would be able to do this in 90 seconds or so but it took him closer to 2 minutes. This is more plausible, the farther back he jumped in. I think he only ran the last quarter mile though, so I doubt this is the explanation.
My guess is that he walked across the line, dressed as a spectator with the back of the pack. If you ever volunteer at a race and you're around the starting line for the back of the pack, there are plenty of spectators standing/wandering around. They were there to cheer on a family or friend/take pictures and then they had off the road once most people have cleared out. No one would think twice about anyone who crossed the starting line in that situation and then ambled off course right away. He then just had to stand around for about 18-19 minutes until a chance presented itself to sneak back in as the course crossed back over the starting line at ~2.8 miles into the race. He could have removed jacket/warm-up pants/whatever during this time. Given that this would be about 21 minutes into the race per gun time there would be plenty of people already done and milling about. It probably wouldn't have been hard to get back onto the course. Problem is, Mike screwed up the math.