Scot Bennett netted an equaliser for the Grecians deep into stoppage time, but only after United's Lee Williamson and Kevin McDonald cancelled out Alan Gow's opener despite Beattie receiving his marching orders for a two-footed lunge on James Dunne with the score at 1-0.
"It was a good way to end the season, although a win would have been better," said Tisdale, whose side will return to life in the fourth tier next season. "We played extremely well at the start of the game and I think 2-2 was probably a fair result.
"We were playing really well and I think this sending off disrupted us really. It should be easier playing against 10 men, but it changes the format of the opposition and their style.
"I feel for James Beattie as he is going to miss the play-offs now. I don't think it would have happened had the referee stamped certain things out earlier on and the game was simmering a little and he let a lot go.
"It was a great way to end the season with a goal with the last kick of the game and Scot Bennett has been one of the big finds of the season for us. He will play a very big part in this team's future over the next two or three years."